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Animal Diversity- I
(Non-Chordates)
Phylum Cnidaria
(Old name- Coelenterata)
Dr. Sukanya Lal
Zoology Department,
Ramjas College,
University of Delhi
Delhi – 110 007
26th June 2007
Page 1 of 115
Chapter index
I. Phylum Cnidaria (Coelenterata): General characters
1. Introduction
2. Morphology
3. Reproduction
II. Classification of Phylum-Cnidaria (Coelenterata)
1.
2.
A.
B.
C.
D.
Introduction
Classification
Class: Hydrozoa
Class Scyphozoa
Class: Cubomedusae
Class: Anthozoa
I Subclass: Alcyonaria (Octocorallia)
II. Subclass: Zoantharia (Hexacorallia)
III. Obelia geniculata
1. Habit and Habitat
2. Structure
A. Polyp
B. Blastostyle
C. Medusa
3. Histological structure of zooids
A. Epidermis
B. Mesogloea
C. Gastrodermis / Endodermis
4. Statocyst
5. Gonads
6. Locomotion
7. Feeding
8. Excretion
9. Respiration
10. Reproduction
A. Asexual mode of reproduction
B. Sexual mode of reproduction
11. Alternation of generation and Metagenesis
12. Polymorphism.
A. Polyps
B. Blastostyle
C. Medusa
Figures 1-10
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IV. Aurelia aurita (Jelly-fish)
1. Introduction
2. Habit and Habitat
3. Structure
A. Oral arms and Mouth
B. Radial canals
C. Velarium
D. Sub-genital pits
E. Gonads
4. Histological structure
A. Epidermis
B. Mesogloea
C. Gastrodermis
5. Nervous System
6. Sense organs
7. Locomotion
8. Water circulation
9. Food and feeding
10. Digestive system
11. Respiration and Excretion
12. Reproduction
13. Life Cycle
A. Planula larva
B. Scyphistoma larva
C. Strobilation
D. Ephyra Larva
E. Metamorphosis
14. Alternation of Generation
Table 1
Figures 1-9
V. Polymorphism
1. Introduction
2. Class- Hydrozoa
A. Order- Hydroida
a. Hydra
b. Obelia
c. Bougainvillea
d. Tubularia
e. Hydractinia
f. Vellela
g. Porpita
B. Modifications of polyp
C. Modifications of medusa
D. Order- Siphonophora
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a. Physalia
b. Diphyes
c. Helistemma
3. Class Anthozoa
a. Pennatula (Sea Pen)
Figures 1-11
VI. Mesenteries
1. Introduction
2. Structure of Metridium (Sea Anemone) explaining Mesenteries
A. Primary mesenteries or complete mesenteries
B. Secondary mesenteries or incomplete mesenteries
3. Octocorallians
4. Hexacorallians
5. Examples of anthozoans showing different arrangement of mesenteries
a. Alcyonium
b. Edwardsia
c. Gonactinia
d. Halcampoides
e. Halcampa
f. Adamsia
g. Haloclava
h. Zoanthus
i. Epizoanthus
j. Cerianthus
k. Antipathes (Black coral)
l. Peachia
m. Metridium
6. Formation of mesenteries
7. Significance and function of mesenteries
Figures 1-17
VII. Corals and coral reefs
1. Introduction
2. Class: Anthozoa
A. Subclass: Octocorallia or Alcyonacea
a. Structure of Octocorallian coral
i. Alcyonium (Dead man’s finger or soft coral)
ii. Heliopora (blue coral)
iii. Tubipora (Organ pipe coral)
iv. Corallium (Red coral)
v. Gorgonia (Sea fan)
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B. Subclass: Hexacorallia (Zoantharia)
a. Structure and formation of a Hexacorallian coral
i. Fungia (Mushroom coral)
ii. Madrepora (Horn coral)
iii. Astraea (Star coral)
iv. Meandrina (Brain coral)
v. Astrangia (white coral)
vi. Antipathes (Black coral)
3. Class: Hydrozoa
i. Millepora (Fire coral or sting coral)
4. Coral reefs
A. Development of coral reefs
B. Types and structure of Coral reefs
a. Fringing reefs
b. Barrier reefs
c. Atoll
C. Theories explaining the formation of coral reefs
a.
b.
c.
d.
e.
Darwin’s Dana subsidence theory
Stutchbury’s volcanic crater theory
Samper Murray solution theory
Submerged bank theory
Daly glacial control theory
5. Significance of corals
6. Coral crisis
A. Natural factors affecting coral growth
a. Temperature
b. Depth of the ocean
c. Availability of light and presence of algae
d. Sea storms
B. Man made factors
a. Increase in human population and development
b. Aesthetic value
c. Sea traffic
d. Predators
C. Coral Bleaching
D. Coral Diseases
Figures 1-18
VIII. Bibliography
IX. Acknowledgements
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I. Phylum Cnidaria (Coelenterata): General characters
1. Introduction: The phylum coelenterata is a Greek word where- ‘coel’ and ‘enteron’
stand for hollow and intestine respectively. It is also known as Cnidaria because of the
presence of a unique organelle called nematocyst or cnidae (formed by special cells
called cnidoblasts). The phylum Coelenterata contains more than 10,000 species,
including jelly fish, fresh water Hydra, box jellies, sea anemones, soft coral and hard
coral forming animals. Cnidarians are Eumetazoans (multicellular) with primarily radial
symmetry. They constitute the lowest group of animals among the Eumetazoa.
Cnidarians can be distinguished from sponges (see phylum- Porifera) as they have a
distinct digestive cavity called coelenteron. They differ from ctenophores (see PhylumCtenophora) by having nematocysts, a polypoid stage and reproduce both sexually and
asexually.
2. Morphology/ Anatomy: These fascinating animals are basically gelatinous in
composition. A majority of them are marine water living except fresh water Hydra and
Craspedacusta. Most of the animals are characterized by radial symmetry about an oralaboral axis. The body presents tissue level organization i.e. cells are organized into
tissues which can perform various functions within an organism. The body wall of a
cnidarian is diploblastic i.e. composed of only two layers of cells, the ectoderm and the
endoderm. A layer of mesogloea generally intervenes between the ectoderm and the
endoderm. Although the mesogloea itself is nonliving and gelatinous, it may contain
living cells derived from embryonic ectoderm. Amoebocytes in the mesogloea of
anthozoans probably play roles in digestion, nutrient transport and storage, wound repair
and antibacterial defense.
Phylum Cnidaria can be divided into four classes: the Hydrozoa (Hydra, Obelia), the
Scyphozoa (jelly fish), Cubomedusae (box jellies), and the Anthozoa (sea anemones,
gorgonians, sea pens, corals etc.). Cnidarians may have two basic body forms- polyps
and medusae (also called zooids) which may be modified into different forms performing
different functions and mutually benefiting each other. Hydrozoans bear polyps or
medusa or both, adult of scyphozoans and class Cubomedusae are only medusoid forms,
while all anthozoans are polypoid forms. Polyps are generally cylindrical, and are found
attached to the main colony or to the substratum while medusae are free living. In polyps
the mouth is surrounded by tentacles and faces upwards i.e. away from the substratum.
In contrast, medusa is umbrella like, having ex-umbrellar and sub-umbrellar surfaces
with the mouth prolonged centrally into a manubrium and facing towards the bottom.
Tentacles are present on the periphery of the sub-umbrellar surface. They freely swim in
water by rhythmic contractions. Their orientation is inverted in relation to that of the
polyps.
In all cnidarians, the mouth leads into a spacious cavity called the coelenteron. The
coelenteron is enclosed by two–layered body wall (diploblastic condition) consisting of
outer ectoderm and inner gastrodersm. Coelenteron is also called the gastrovascular
cavity as it acts both as a digestive system and a circulatory system for the circulation of
food, oxygen, excretory wastes and many other materials with in the body. It is lined by
gastrodermis (also known as the endodermis) which is sometimes infolded to form radial
septa or mesenteries. These mesenteries increase the surface area over which digestion
may take place, because the primary function of the endoderm is digestion. Cnidarians
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are mainly carnivorous although some soft corals have been found to feed also on
phytoplankton. Colonial forms are fixed, so they cannot move actively from one place to
another. They capture their prey with the help of tentacles encircling their mouth.
Special stinging cells or cnidoblasts unique to this phylum are present abundantly on the
tentacles and at the terminal ends of the mesenteries in anthozoans. Three general types
of cnidoblasts are present in different cnidarians. Cnidoblasts having a cnidocil and
nematocyst, which is a common type of stinging cells, is present in hydrozoans and
scyphozoans. Besides this majority of anthozoans have spirocysts or ptychocysts.
Spirocyst, as its name suggests, contains a spirally coiled thread inside. This thread, on
discharge, releases sticky material. Spirocysts are present in most of the anthozoans in
addition to nematocysts. Similarly, ptychocysts contain a tubule, which is haphazardly
arranged inside the capsule, and is released. Ptychocysts are present in the tube dwelling
anemones like Cerianthus. These can be discharged with great force for a variety of
functions. Around 30 different types of cnidoblasts are reported to be present in different
cnidarians. Many cnidarians have symbiotic photosynthetic algae i.e. zoochlorallae (in
Hydra) and zooxanthellae (in marine cnidarians) within their gastrodermal cells or in
their ectodermal cells. They live symbiotically with these algae and obtain additional
nutrients from them. They benefit algae in turn by providing CO2 and metabolic products
for photosynthesis.
The nervous system is very primitive and lacks a central nervous system. In contrast it
consists of a network of nerve cells and nerve processes which generally synapse on one
another repeatedly and terminate at a neuromuscular junction. This type of a nerve
network helps in immediate spread of excitation over the entire body of the animal when
sensory cells are stimulated. As a result, the animal can change its orientation or react
according to the situation. Muscle layers are derived from epitheliomuscular cells and
endotheliomuscular cells that possess elongated, contractile bases anchored in the
mesogloea. Circulatory, excretory or respiratory organs are completely lacking as the
functions of these systems are performed by the gastrovascular cavity.
A majority of the animals belonging to the class hydrozoa exhibit the phenomenon of
polymorphism, which is defined as the occurrence of structurally and functionally
different forms within the same organism during its life cycle. Different types of
individuals are called zooids e.g. Obelia has polyps called feeding zooids or gastrozooids,
while free living reproductive zooids are called medusae. Many anthozoan polyps
secrete a calcareous skeleton made up of calcium carbonate (externally or internally),
forming exoskeleton or endoskeleton respectively. This helps in deposition of calcareous
skeletons called corals which helps in the formation of massive deposits in the sea called
coral reefs.
3.
Reproduction:
Reproduction in cnidarians occurs asexually by budding,
fragmentation or by pedal laceration, while sexual reproduction takes place by the sex
cells produced by gonads. In some animals like Hydra, which have only a polypoid
stage, gonads are developed from the epidermal cells to reproduce sexually in addition to
the asexual reproduction by budding. Aurelia exists only as medusa; while Obelia has a
characteristic life cycle, in which polyp and medusa alternates with each other during
their life cycle. Medusa being a reproductive zooid carries four gonads, one of which is
located along each of the radial canals. When the gametes mature, they are released by
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rupturing of the gonad wall. Sperms fertilize the ova in the sea water and the zygote
leads to the formation of a two layered, characteristic larval stage called the Planula larva.
Planula larva has a ciliated ectoderm and freely swims in the water for some time before
it settles and develops into a polypoid form. This polypoid form is tubular; bears the
mouth and tentacles directed upwards and are attached to the substratum aborally. It
multiplies asexually and either produces other polyps, which may lead an independent
life to form a colony, or gives rise to free living medusoid stage bearing gonads.
Hydrozoans thus exhibit a phenomenon of metagenesis i.e. an alternation of asexual and
sexual phases during its life history.
II. Classification of Phylum-Cnidaria (Coelenterata)
1. Introduction:
i.
They exhibit radial symmetry and are acoelomates (without coelom).
ii.
Tissue level (cells are organized into tissues and perform different
functions) of organization is achieved in this lowest group of animals
among the true metazoans.
iii.
Body wall is diploblastic (it is made up of two layers of cells, an outer
ectoderm and inner gastroderm with a thin layer of mesogloea secreted
by both these layers).
iv.
Special stinging cells or cnidoblasts, unique to this phylum are present.
v.
A single gastrovascular cavity (coelenteron), opening by a mouth
(single opening) that serves as a mouth and anus.
vi.
Nervous system is in the form of a net work of nerve cells and nerve
fibers present in the mesogloea.
vii.
Organism may be a sedentary (fixed) polyp, free living solitary
medusa, or may bear both polyp and medusa in the same colony.
viii.
Asexual reproduction takes place by budding or fragmentation.
ix.
Sexual reproduction produces a characteristic, free living, ciliated
planula larva.
x.
A phenomenon of Polymorphism is exhibited in hydrozoans and
anthozoans..
xi.
Many organisms form corals and help in the formation of coral reefs (a
unique aquatic ecosystem).
2. Classification:
A. Class: Hydrozoa:
i. A majority of the hydrozoans are colonial and found in marine water
except Hydra and few others which are fresh water living animals.
ii. In solitary and colonial forms, polyps are predominantly nutritive
zooids.
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iii. Sexual zooids are called medusae and are produced by asexual budding.
These are umbrella shaped and free swimming
iv. Gonads are ectodermal and are borne by the medusa. However, in
Hydra like forms, these are borne by polyps as they lack medusoid
stage.
v. Medusa possesses a circular shelf within the umbrella called velum.
Hydrozoans are classified into following orders:
a. Order: Trachylina
i. All marine.
ii. Contains most primitive animals of the class hydrozoa.
iii. No polypoid stage, only medusoid zooids present.
iv. Bell margin smooth.
v. Gonads on radial canals or attached to manubrium.
vi. Sense organs are tentaculocysts (modified tentacles) which are partly
mesodermal in origin. Ocelli usually absent.
vii. Planula larva develops into an actinula larva which directly develops
into a medusa without intervening polyp stage.
Examples: Aglaura, Liriope, etc.
b. Order: Hydroida
i. Solitary or colonial.
ii. Well developed polypoid generation without gastric ridges.
iii. Medusoid stage present or absent.
iv. Velum is present in the medusa.
v. Sense organs of medusa are ocelli and statocysts which are ectodermal
in origin.
Suborder 1: Limnomedusae
i.
Majority are fresh water animals.
ii.
Small solitary polyps present and medusae bud off from the sides.
Examples: Craspedacusta
Suborder 2: Anthomedusae (Athecate)
i.Solitary or colonial hydrozoans.
ii.Bell shaped, free medusa are common.
iii.Polyp athecate (not surrounded by skeletal covering).
iv.Statocyst absent.
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Examples: Eudendrium, Hydractinia, Tubularia, Pennaria etc.
Suborder 3: Leptomedusae (Thecate)
i.
Colonial hydrozoans.
ii.
Polyps are thecate i.e. they are enclosed by hydrotheca, and medusa
produced by blastostyles is covered by gonotheca.
iii.
Free medusae are generally absent, and when present, are umbrella
shaped and bear statocysts for equilibrium.
Examples: Obelia, Companularia, Sertularia, Plumularia etc.
Suborder 4: Chondrophora
i.
Pelagic, free swimming forms.
ii.
Polymorphic polypoid colonies.
iii.
These animals can be considered as large, single, inverted polyps.
Examples: Vellela (float thin, with erect sail), Porpita (float disc shaped).
c. Order: Actinulida
i.
Very small, solitary hydrozoans like actinula larva.
ii. Medusoid stage is completely absent.
Examples: Otohydra, Halammohydra.
d. Order: Siphonophora
i. Marine, pelagic, especially in warm seas.
ii. Free swimming hydrozoan colonies comprising of polypoid and medusoid
zooids.
iii. Upper end of colony usually bears a supporting float or swimming bells.
iv. Nematocysts many, large and powerful.
v. Medusae incomplete, attached to stem or disc, rarely free.
vi. Gonads in gonophores, which are not set free. Examples: Physalia
(Portuguese Man of war, float inflated), Stephalia, Nectalia.
e. Order: Hydrocorallina
i. Colonial, polypoid hydrozoans.
ii. Polyps of 2 forms, gastrozooids and dactylozooids.
iii. Hydromedusae consisting of coenosarcal canals, ectoderm of which
secretes a hard
iv. Calcareous skeleton filling up the spaces of coenosarcal mesh work.
v. Rudimentary medusae in the form of gonophores are generally
present. Defensive and feeding polyps located within star-shaped
openings on the skeleton.
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Example: Millepora.
B. Class Scyphozoa:
i. All marine jelly fishes.
ii. Chiefly free swimming medusae, bell or umbrella form, with strong 4 per
radial symmetry and much gelatinous mesogloea.
iii. No true velum, no stomodeum.
iv. Gastric ridges present about mouth.
v. Central gastrovascular stomach and radial canals with complex branches
are present.
vi. Notches in bell margin with sense organs rhopalia (tentaculocysts) having
endodermal statoliths.
vii. Medusae sexual, dioecious with endodermal gonads in pouches of the
gastric cavity.
viii. Polyp generation none or reduced (hytratuba and scyphistoma) producing
medusae directly or by transverse fission.
a. Order: Stauromedusae
i. Sessile polypoid forms attached by a stalk on the aboral side of the
trumpet shaped body.
ii. Marginal sense organs (rhopalia) absent but 8 simple tentacles present.
iii. Four cornered mouth with small oral lobes and manubrium is present.
Examples: Haliclystus: Lucernaria etc.
b. Order: Coronatae
i.
Free swimming scyphozoans found in warm waters.
ii.
Body cubical with four flat sides. Bell surrounded by circular furrow,
above scalloped margin.
iii.
Four hollow inter-radial tentacles, four rhopalia at per radial positions.
iv.
Found mainly in deep waters.
Examples: Periphylla, Nausithoe, Linuche, Atolla etc.
c. Order: Semaeostomeae
i Saucer-shaped bells having scalloped margins bearing tentacles.
ii Mouth central, corners prolonged as four oral frilly arms.
iii Gastrovascular cavity extends from central stomach to radial canals and
circular canal at the bell margin.
iv Chiefly in coastal waters
Examples: Aurelia, Cynea, Pelagia, Aurelia, Stygiomedusa etc.
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d. Order: Rhizostomeae
i.
With 8 large adradial, root like simple or branched oral arms with numerous
suctorial mouths
ii.
Original central mouth is lost because of fusion of oral arms except in
Stomolophus.
iii.
Gastrovascular cavity without septa.
iv.
Eight or more rhopalia (tentaculocysts), four are per-radial and 4 ad-radial.
Examples: Cassiopea, Rhizostoma, Mastigias, Stomolophus.
C. Class: Cubomedusae: Medusoid forms, with bell cubical and margin bent inward.
i. Known as box jellies with complex eyes and potent toxin.
ii. Tentacles 4 or in 4 groups.
iii. Feed mostly on fish.
iv. Planula larva develops into a fixed polypoid larva which metamorphoses into
medusa.
v. Found in tropical and subtropical waters, shores and open seas.
Examples: Carybdea, Chironex, Chiropsalmus etc.
D. Class: Anthozoa: All marine, solitary or colonial, includes sea anemones, corals
and sea pens etc.
i. All polypoid forms, sessile (fixed), solitary or colonial.
ii. Oral disc flat, with hollow tentacles.
iii. Mouth leading into stomodeum (gullet) lined by ectoderm, usually with ciliated
groove (siphonoglyph) in the pharyngeal wall leading from mouth.
iv. Enteron divided by vertical septa (mesenteries) bearing nematocysts on inner
margins.
v. Nematocysts do not possess an operculum like hydrozoans and scyphozoans and
are called spirocysts or ptycocysts.
vi. Mesogloea a connective tissue.
vii. With or without skeleton.
viii. Gonads (endodermal) in septa or mesenteries.
ix. Planula larva develops from the fertilized egg and metamorphoses to form another
polyp.
x. Many anthozoans reproduce asexually by longitudinal or transverse fission.
xi. Pedal laceration (in which parts of the pedal disc detach from the rest of the body
and form a new individual) is also present.
xii. Majority forms corals.
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I Subclass: Alcyonaria (Octocorallia)
i. With 8 pinnately branched tentacles and 8 single complete mesenteries or
septa.
ii. Modular colonies and are often polymorphic.
iii. One central siphonoglyph is present.
iv. Soft corals, polyps are supported by calcareous endoskeleton of spicules
secreted by cells in the mesogloea.
a. Order: Stolonifera
i. Polyps arising separately from common stolon or mat.
ii. Skeleton of separate calcareous spicules.
iii. Sometimes fused as tubes.
iv. Clavularia in California coast.
v. Tubipora musica, organ pipe coral, in warm waters on coral reefs.
Examples: Clavularia, Tubipora etc.
b. Order: Telestacea
i. Colony consists of simple or branched stems bearing lateral polyps
ii. Skeleton consists of calcareous spicules.
Example: Telesto.
c. Order: Alcyonacea (soft corals)
i. Polyps with lower parts fused in a fleshy mass and only oral ends protruding out.
ii. Polyps are dimorphic in some forms bearing autozooids and siphonozooids.
iii. Skeleton of separate limy spicules.
iv. Found mostly in warm shore waters.
Examples: Alcyonium, Gersemia, Anthomastus, Sarcophyton etc.
d. Order: Helioporacea
i.
Skeleton massive made up of calcareous fibers.
ii.
Commonly known as blue corals.
Examples: Heliopora, blue coral of Indo-Pacific region.
e. Order: Gorgonacea
i. Colony usually plant-like arising from a basal disc.
ii. Axial skeleton of calcareous spicules made up of horn like gorgonin or both.
iii. Polyp short.
Examples: Corallium (red coral), used for jewelry; Gorgonia (Sea fan)
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f. Order: Pennatulacea
i. Colony fleshy, of one long axial polyp, with many dimorphic polyps arranged on
both sides present on bare stalks.
ii. Skeleton of limy spicules.
Examples: Pennatula (Sea pen) feather like; Renilla (sea pansy), disc shaped;
Umbellula.
II. Subclass: Zoantharia (Hexacorallia)
i.
Tentacles few to many (usually in the multiples of six) and 6 pairs of primary
mesenteries.
ii. Many species are solitary and lack any protective covering (sea anemone), some
are colonial but never polymorphic(true stony corals or scleractinian corals)
iii. True stony corals secrete hard calcium carbonate exoskeleton. They may be reef
building (hermatypic) or not (ahermatypic).
iv. Siphonoglyph 1 or 2, associated with the pharynx.
v. Many have endosymbiotic relationship with unicellular, photosynthetic
zooxanthellae.
a. Order: Zoanthidea
i.
No skeleton or pedal disc.
ii. Polyps usually united by basal stolons.
iii. Some solitary with stalked base
iv. Many species on exterior of various invertebrates.
Example: Zoanthus, Epizoanthus on hermit crab, Parazoanthus.
b. Order: Actinaria
i.
All sea anemones.
ii.
No skeleton, polyps of same size, columnar with muscular wall and usually a
pedal disc.
iii.
Stomodeum usually with siphonoglyph.
iv.
Mesenteries paired, often in multiples of 6.
v.
Found on rocks, on sand, or on invertebrates.
vi.
Sessile, but not fixed.
vii. Essentially solitary.
viii. Filaments with ciliated areas
Examples: Halcampoides, Metridium, Anthopleura, Adamsia on hermit crab shells,
Edwardsia in burrows.
c. Order: Madreporaria or Scleractinia (Stony corals).
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i.
Exoskeleton compact, calcareous, polyp small or minute in cups on skeleton.
ii. Tentacles in the multiples of six.
iii. No siphonoglyph.
iv. Muscles feeble.
v.
Mostly colonial in warm seas.
Examples: Fungia (solitary), Acropora, Astrangia, Oculina.
d. Order: Corallimorpharia
i. No ciliated areas on filaments.
ii. With capitate tentacles.
iii. Usually in radial series.
Examples: Corynactis, Ricordea.
e. Order: Ceriantharia
i. Solitary, slender, elongate, anemone like, living in vertical tubes.
ii. Many tentacles in 2 circles.
iii. No pedal disc, one siphonoglyph.
iv. Solitary.
Example: Cerianthus inhabits slime-lined vertical tubes in sea bottom and take out
their tentacles and oral disc from the tube at the time of feeding.
f. Order: Antipatharia (Black corals)
i. Skeleton plant like, stems (some branched) composed of horny material and bear
small polyps.
ii. Tentacles 6 in number.
iii. Found in deeper tropical waters.
Example: Antipathes
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III. Obelia geniculata (Sea fur)
1. Habit and Habitat:
A typical example of class hydrozoa is Obelia geniculata. It is a colonial hydrozoan,
commonly known as sea fur and is found attached to rocks, wooden piles, shells or
seaweeds. The colony grows by budding from a single hydra-like individual.
It consists of individuals (zooids) of three different forms-polyp, blastostyle and medusa
attached to vertical branches. There are many polyps grown on the branching colony
with the blastostyles grown in the axils of lower branches of the colony. Each blastostyle
is a clubbed shaped extension of the main colony which by asexual budding gives rise to
many small saucer shaped medusae. Each medusa is a free living form of the colony. It
provides a means of dispersal and is able to reproduce the colony sexually.
Obelia is one of the important cnidarians as it exhibits the phenomenon of polymorphism
and alternation of generation also called as metagenesis.
2. Structure:
Obelia is a microscopic, delicate and fur like colonial animal. It is whitish or brown in
color. Its colony consists of zigzag stems arising from horizontal, hollow, branched, root
like filaments called Hydrorhiza (Fig. 1). It forms a complicated meshwork over the
surface of the substratum and helps in fixing the colony to the substratum (sedentary or
fixed nature). From the horizontally running hydrorhiza, several, vertical branches arise
called hydrocauli (hydrocaulus singular). Each hydrocaulus forms the main stem of a
colony and is of 2 to 3 cm in height. It grows upright having alternate branching system.
Each branch terminates into an individual (also called zooid) of a colony called polyp or
a gastrozooid. Gastrozooid is so named as its main function is to feed the colony. Some
of these branches may end up in the form of small buds representing young forms of the
polyps. From the axils of each older branch arise cylindrical zooids, called blastostyle
which are generally present towards the lower regions of the colony. Blastostyles, by
asexual budding give rise to sexual zooids called medusae. Medusae are called sexual
zooids because each one of them bears four gonads which are located on the per-radial
position. When a medusa gets mature, it is released free from the blastostyle and swims
freely in the water to carry out the process of sexual reproduction.
The entire colony of the Obelia including its branches and zooids are covered by a
chitinous external skeleton called perisarc and the innermost layer is called coenosarc.
Coenosarc encloses a cavity within called coenosarcal canal. Basically, coenosarc
consists of an external epidermis and an internal layer of gastrodermis with a thin sheet of
noncellular mesogloea sandwiched in between them. As coenosarcal canal is continuous
with the zooids and the colony, digested food, oxygen and even water is circulated and
distributed through it to all the parts of the colony. Perisarc is tough, transparent, noncellular and cuticular covering. It is secreted by the epidermis which is present beneath
it. It provides rigidity and physical strength to the colony. Perisarc was in direct contact
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with the coenosarc in the beginning of development but gets separated from the
coenosarc by a space, except at certain points at regular intervals. Because of the rigidity
of the perisarc, colony and its branches cannot move except at certain points where
circular rings or annuli formed by perisarc are present. The perisarc surrounding the
polyp is called hydrotheca while surrounding the blastostyle is called gonotheca.
A. Polyp: Polyp is also called as hydranth. Majority of the individuals of the Obelia
colony are polyps. These are meant for the feeding of the colony and thus known as
gastrozooids (Fig. 1 and 2). Number of polyps is more in Obelia colony as compared to
blastostyle. Polyps resemble Hydra structurally. These are present at the terminal ends of
almost all the branches of the colony. One end of the polyp is connected to the
hydrocaulus while its free end is produced into conical structure, the manubrium
(hypostome), which is around one third of the length of the polyp. Manubrium bears a
small opening in the center called mouth. Mouth leads into a big cavity called
gastrovascular cavity which is continuous within the body of polyp and with the
coenosarcal canal of the hydrocaulus.
Around the base of the manubrium are present numerous tentacles in a single circlet.
Tentacles are long, pointed, and are projected beyond the manubrium. They bear
batteries of nematocysts in their ectodermal cells. In Obelia tentacles are solid because
of the presence of a single row of cylindrical and highly vacuolated endodermal cells. In
contrast, tentacles of Hydra are hollow.
Perisarc present on the outer surface of the polyp expands and forms a loose, protective
conical, vase like structure around the polyp called hydrotheca. It is transparent and has a
smooth outer margin. At the base of the polyp, hydrotheca is produced into a ring like
horizontal platform or shelf. At the time of danger, polyp can retract suddenly by folding
its tentacles within the hydrotheca and shelf checks the polyp from being pulled within
the perisarc of the hydrocaulus. Perisarc forms three to four annuli at the base of the
hydrotheca surrounding the polyp. These annuli help in swaying movements of the polyp
to capture the prey. The basal disc of the polyp is continuous with the coenosarc of the
stem to which it is attached. Though, each polyp is an individual, all the polyps are
connected together by a tubular coenosarcal canal, through which the coelenteron
continues.
B. Blastostyle: Blastostyle is a zooid which asexually produces numerous medusa buds
from its upper end. Blastostyles are produced in the axils of the vertical branches
(hydrocauli). These are normally present towards the basal (older) branches of the colony
(Fig. 1 and 3a). The number of blastostyle is less as compared to polyps in a colony.
Every blastostyle consists of a central stem arising from the coenosarc. It is devoid of
mouth and tentacles and has a reduced gastrovascular cavity, it cannot feed on itself,
however, nutrients are supplied through the coenosarcal connection with the gastrozooid.
It is enclosed in a long, cylindrical, transparent covering of perisarc called as the
gonotheca. Gonotheca has a small opening, the gonopore towards its terminal end
through which medusae can come out. Blastostyle with the gonotheca around is called
Gonangium. The basal end of gonangium has two or three annular rings which provide
flexibility to it as in the case of polyps. When medusae buds are fully formed, they are
released out through gonopore into the surrounding water to lead a free swimming life.
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C. Medusa: Medusa is a gonozooid which is produced by asexual budding from
blastostyle. Mature medusae are present towards the terminal end of the blastostyle and
younger ones are present towards the base of the blastostyle (Fig. 3b, c, and d). Each
medusa carry four gonads, each one is located along each of the four per-radial canals on
the subumbrellar side. When the gonads become ripe, they shed gametes by rupturing
their wall. Medusae are produced from blastostyles in large numbers during spring and
summer season.
Medusa has an umbrella like structure, with the convex, exumbrellar surface and a
ventral subumbrellar surface. The edge of the umbrella is produced inwards as a short
double fold of integument to form a diaphragm like structure called velum. A medusa
having velum is called craspedote and when velum is absent, it is called acraspedote
(Aurelia). The rim of the medusa bears highly contractile tentacles with batteries of
nematoblasts. A young medusa has only sixteen tentacles and the number increases as it
grows. From the subumbrellar surface of the medusa arises a short hollow structure
called manubrium bearing a four sided mouth at its terminal end. Manubrium appears
like the handle of the typical umbrella.
Medusa has a radial symmetry. If a circle is drawn with a four sided mouth in the center,
then four radii arising from the angles of the mouth are named as per-radial positions..
The tentacles present at the per-radial positions are called per-radial tentacles. Bisecting
the angle between two per-radii is known as inter-radial position. Radius passing through
per-radial position and inter-radial position is named as ad-radial position. The tentacles
present at inter-radial and the ad-radial position are called inter-radial and ad-radial
tentacles respectively.
A young medusa has four per-radial, four inter-radial and eight ad-radial tentacles. The
mouth leads into a short gullet in the manubrium followed by a stomach occupying the
central position of the subumbrellar surface of the medusa. The stomach continues into
four thin, delicate radial canals which run towards the periphery of the medusa and are
placed at equal distance from each other (Fig. 3d). At periphery of the medusa, the four
radial canals open into a circular canal which runs all along the margin of the medusa. In
medusa gastrovascular cavity is thus restricted to gullet, stomach, radial canals and
circular canal.
3. Histological structure of zooids:
The basic histological structure of polyp,
blastostyle, medusa and coenosarc are similar and resembles Hydra. Therefore, the body
wall of Hydra is discussed here briefly to understand the histology of the zooids. The
body wall is made up of two layers of cells and is diploblastic. The outer and inner layers
are known as ectoderm (epidermis) and endoderm (gastrodermis). A thin, delicate,
transparent and noncellular material called mesogloea secreted by both epidermis and
gastrodermis lies in between these layers.
Medusa has an outer epidermis, and innermost gastrodermis with
a thin layer of
intervening mesogloea. Both exumbrellar and subumbrellar surfaces are covered with
epidermis. The whole canal system is lined by a thin sheet of gastrodermis called as
gastrodermal core, which is formed by the fusion of an upper and a lower layer of
gastrodermis. Velum is composed of a double layer of epidermis enclosing a thin layer
of mesogloea, but without gastrodermis. Tentacles are solid because of the presence of
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vacuolated gastrodermal cells that are covered externally by epidermis bearing batteries
of nematocysts.
Exumbrellar surface (as it always faces up) has a very thick gelatinous mesogloea present
between epidermis and gastrodermis and forms the main bulk of the body. Mesogloea
contains lot of water and noncellular fibers. Muscular regions of the epitheliomuscular
cells are better developed and are arranged in circular, longitudinal and radial tracts
forming a well formed musculature. Tentacles have swollen regions at their bases
because of the presence of interstitial cells. Interstitial cells are totipotent as they give
rise to formation of any type of the cells depending upon the requirement of the
individual tissues. These cells also replace cnidoblasts or nematoblasts (cells bearing
nematocysts) which die soon after the discharge of the nematocyst. Nematoblasts are
quite numerous on the margin of umbrella, tentacles and around the mouth. Nematocysts
borne by nematoblasts help in paralyzing the prey and also protecting the colony by
keeping away the enemies (see details below).
Nerve cells and fibers are concentrated around the margin of the cell forming two nerve
rings, inner and outer. The ex-umbrellar or outer nerve ring supplies the tentacles, while
the subumbrellar nerve ring or inner nerve ring supplies the subumbrellar musculature
and the statocysts. Inner nerve rings around the bell margin is associated with the
rhythmic contractions of the medusa.
A. Epidermis: Epidermis is thin and is made up of many different types of cells which
are structurally and functionally differentiated (Fig.4).
a. Epithelio-muscular cells: Each epithelio-muscular cell has outer broader end and
narrow inner ends prolonged into unstriped muscle fibers. These cells are so named as
they form the outer epidermis on one side and have muscular ends on the inner side,
which are arranged parallel to the long axis of the body. Their contractions make the
body and tentacles shorter and thicker.
b. Sensory cells: Sensory cells are distributed all along the external surface of the body
in between the epidermal cells. Their number is more on the tentacles and manubrium.
They bear a delicate hair like sensory cilia at their free ends while their inner ends are
withdrawn into fine fibers which meet the nerve net present below the epidermis.
Sensory cells receive and transmit impulses from and to the nerve cells. These are
sensitive to touch, temperature and chemicals and accordingly modified to perform
different functions.
c. Cnidoblast (nematoblast): Some of the cells of epidermis are specialized for the
protection of the colony. These cells are called the cnidoblasts or nematoblasts and are
specifically present only in coelenterates. Cnidoblasts are also known as stinging cells.
These cells are aggregated in more numbers in the epidermis of the tentacles and such
regions are called batteries of nematocysts (Fig. 5a).
i. Structure: Each cnidoblast is a large cell enclosing a pear shaped cell called
nematocyst. Its nucleus lies on one side of the cell. Nematocyst has a central cavity
containing poisonous fluid, the hypnotoxin and is closed with a lid called operculum.
Poisonous fluid or hypnotoxin is a mixture of proteins and phenol. The outer end of the
nematocyst invaginates into a funnel shaped structure tapering into a coiled thread like
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filament folded like a spring inside the cavity (Fig. 5b), the tip of the thread may be open
or closed but the base is swollen to form the butt or shaft. There are three large spines on
the butt; pointing inwards are called stylets or barbs. In addition, three spiral rows of
minute spines called barbules or spinnerets are also present. There is few supporting
muscle fibers arising from the basal region of the cnidoblast and are attached to the
nematocyst. A small, pointed, sensory hair like structure called cnidocil is present close
to the operculum and is projected outside beyond the epidermis.
ii. Function: Cnidocil is sensitive to vibrations in the water and when a microorganism
or prey passes close to the cnidarians, it stimulates the nematocyst to discharge (Fig: 5c).
When this happens, the central cavity containing hypnotoxin is intensely compressed by
the muscle fibers and the operculum opens up immediately. As a result, the coiled thread
opens out with a great force turning inside out. As the thread is shot out, barbs and
barbules penetrate the prey so that the hypnotoxin is injected and the prey is killed by the
poison. Batteries of nematocysts are found on the tentacles, which all discharge together
and paralyze or kill the enemy or prey. Nematocysts thus help not only in protection
from enemies but also in capturing food. Some nematocysts can produce a burning
sensation, itching or even death to higher animals.
iii. Types of nematocysts: There are different types of nematocysts (Fig: 6a) present in
different cnidarians performing different type of function. In fact all of them are known
as stinging cells.
- Penetrant type: It is the largest type of the cnidoblast and is present in Hydra. The tip
of the thread is open and penetrates the tissues of the prey and injects poisonous fluid
once it is discharged (Fig. 6b).
- Volvent type: It is pear shaped. The tip of the thread is closed and is mainly used to
hold the prey by coiling around it.
- The Glutinants: They are of adhesive types and used by Hydra to stick its tentacles to
the substratum when the Hydra is moving by looping.
- The Streptoline Glutinant: It is oval and its thread bears minute spine like structures
and they get coiled on the prey, once they are discharged.
- The Stereoline glutinant: They discharge a straight unarmed thread.
All these types of the nematocysts are discharged only once and then these cells die and
are replaced by new cnidoblasts formed by interstitial cells.
Spirocysts and ptychocysts are some of the other types of nematocysts which are present
only in anthozoans (sea anemones and coral forming animals).
- Spirocysts: Sea anemones bear spirocysts in their ectoderm and endodermal cells in
addition to nematocysts. In their tentacles spirocysts are more in number as compared to
nematocysts. Spirocysts consists of capsule with a single wall and a long thread. They
help the anemone to capturing the food organisms that have hard surfaces e.g. small
crustaceans attached to the substratum.
-Ptychocysts: Large anemones like Cerianthus and Zoanthus which are solitary and tube
dwelling, bear a nematocyst like organelle in their ectoderm. These animals are adapted
to stay in the tubes lined by mucous and discharged threads and capsules of ptychocysts.
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d. Interstitial cells (Totipotent cells): These cells are located in between the adjacent
epitheliomuscular cells. These are small rounded cells having prominent nuclei with
little cytoplasm. These cells are also called totipotent cells as they can differentiate into
any type of cell depending upon the requirement of the body tissues.
e. Nerve cells: There are both epidermal and gastrodermal nerve nets in cnidarians.
Nerve cells are located beneath the epitheliomuscular cells and musculo-nutritive cells
and lie parallel to both the layers. Each nerve cell consists of a nucleated cell with two to
many processes called nerve fibers. These nerve cells are similar to multipolar neurons
of higher animals but are very primitive in nature as they appeared for the first time in the
coelenterates. The nerve cells form an irregular nerve net and synapse with other nerve
cells. The network of nerve cells and fibers is dense in the region of the oral disc and the
tentacles. In addition, nerve cells are in contact with the basal parts of the sensory cells
of the epidermis and gastrodermis.
f. Gland cells: These cells secrete mucus which helps in adhesion, food uptake and
protection. In Hydra, their number is more at the basal disc for the attachment and for
locomotion.
g. Sex cells: During breeding season, interstitial cells of epidermis give rise to testis or
ovary. Epidermal gonads are present only in the class hydrozoa. In scyphozoans and in
anthozoans, gonads originate from the gastrodermis or endodermis.
B. Mesogloea: It is very thin in Hydra and Obelia colony except medusa where it is thin
only on the subumbrellar surface and very thick on the exumbrellar surface of the medusa
which faces upwards. Nerve nets related to epidermis and gastrodermis are located
towards the mesogloea.
C. Gastrodermis / Endodermis: Gastrovascular cavity is lined throughout by the
endodermis in hydrozoans. In Anthozoans, gastrodermis extends in the form of folds
called mesenteries or septa with a thin sheet of mesogloea within. Types of cells present
in the gastrodermis are:
a. Musculo-Nutritive cells: Gastrodermis consist of long, columnar cells called
musculo-nutritive cells. Some of the cells bear flagella to maintain a constant
circulation of fluid and food within the gastrodermal cavity and within the
coenosarcal canal. There are certain cells which form pseudopodia like structures
to ingest food particles by phagocytic activity. Intracellular digestion within these
cells completes the digestive process. Near the manubrium, these cells are modified
into circular muscles to close and open the mouth.
b. Gland cells: These are present within the gastrodermis, which secrete digestive
enzymes within the gastrovascular cavity for the extracellular digestion of food.
c. Nerve cells: These cells are like epidermal nerve cells and form a nerve net
underneath the gastrodermis parallel to the mesogloea.
d. Cnidoblasts: These cells are absent in the gastrodermis but some of them are also
present in gastrodermis of mesenteries in anthozoans.
4. Statocyst: There are eight receptor organs i.e. statocysts located at the bases of eight
ad-radial tentacles on the subumbrellar surface, just inside the margins of the bell (Fig.
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3d). Each statocyst consists of a small fluid-filled sac which contains a movable, round
particle of calcium carbonate, called statolith or otolith. Statolith is secreted by a large
cell, the lithocyte. The statocyst is covered by a layer of epithelial cells which are
sensory in nature and connected at the base with the nerve cells (Fig. 7a and b). The
other free ends of these sensory cells have protoplasmic processes which touches the
statolith. These help in maintaining the body position of the medusa in a right position
while floating in the water, i.e. exumbrellar surface up and subumbrellar surface down.
Besides there are two circular nerve rings; nerve ring I (exumbrellar nerve ring) and
nerve ring II (subumbrellar nerve ring) which are simply the aggregations of the nerve
cells and are present around the margin of the medusa. Nerve ring I supplies the nerves to
the tentacles while nerve ring II supplies nerves to the subumbrellar musculature and the
statocysts. When medusa is swimming and the body gets tilted, the statolith falls away
from the processes of sensory cells. With the result nerve impulse is created in the
sensory cells and transmitted to the nerve ring II and then to the nerve net present nearby.
Epitheliomuscular cells of the stimulated side contract immediately and the medusa is
brought back to its original position. Statocysts control the swimming movements of the
medusa in the water and thus help in maintaining equilibrium by muscular coordination.
5. Gonads: Medusa is a sexual zooid as it bears gonads for the sexual reproduction (Fig.
3c and 3d). There is no sexual dimorphism between male and female medusa but it is
certain that one medusa can behave either as male and will produce only sperms while
another medusa acts as female by producing ova.
Testis and ovaries are borne by different medusae, so they are dioecious. Each medusa
bears four gonads which are located in the middle of each radial canal on the
subumbrellar surface, so the position of gonads is per-radial. They can be easily noticed
as bulging on the subumbrellar surface almost at equal distances between the manubrium
and the velum (Fig. 3e). Once the medusa is released from the blastostyle through
gonopore of gonotheca, the gonads get matured afterwards.
Each gonad is an ovoid
body with an outer epidermis which is present in continuation with the epidermis of the
subumbrellar surface. It has an inner layer of gastrodermis which is continuous with that
of the radial canal. The space between two layers is filled with mesogloea containing
interstitial cells which may either differentiate into sperms or ova at the time of maturity.
Germ cells are originated from the epidermis of the manubrium when it was attached to
the blastostyle. Later on, these cells are transferred through gastrodermis into the gullet
and then reach the gonads. When the gonads mature, epidermis ruptures and the sperms
and ova are released in the water respectively. Sperms are minute and flagellated with
the help of which they actively swim in the water. While the ova are large rounded cells
which may either be released in the water for fertilization or may remain within the
gonads and are fertilized by the incoming sperm there itself.
6. Locomotion: As Obelia colony is sedentary (fixed), so it cannot move from one
place to another. There are annuli present at the base of the polyp and blastostyles which
help them to undergo sideways movements and provide little flexibility to the colony.
The polyps and their tentacles can undergo contraction and relaxation due to the
epitheliomuscular cells present in the epidermis. Medusa is a free living zooid, once it is
released from the blastostyle; it floats in the water with its exumbrellar surface upwards
and subumbrellar surface facing down. It actively swims by the muscular action which
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brings about opening and closing of the bell. This propels the water behind and the
animal moves forward. They also float in the water passively by the water currents and
winds blowing over the sea. At times their body may be disturbed or tilted with upside
down or turned inside out. Statocysts present at the periphery help in maintaining the
balance and position of the medusa.
7. Feeding: Polyps are called gastrozooids as they feed the whole colony. They are
carnivorous and feed on microorganisms like nematodes, worms, crustaceans, small fish
etc. The food is captured with the help of tentacles bearing batteries of nematocysts.
Nematocysts kill the potential food and tentacles get the food within the mouth. Gland
cells present in the gastrodermis release digestive juices within the gastrovascular cavity
which break the food into smaller pieces and partly digest the food. Partly digested food
is taken inside the musculo-nutritive cells and digested within the cells. Digested food is
absorbed and circulated throughout the colony by the flagellated cells present in the
gastrodermis.
Medusa being free swimming can feed them, once they are released in the water. In
medusa, the food is captured by the highly extensible mouth and then taken into gullet.
From the gullet it is transferred to the stomach where digestive enzymes are released and
digested food is then circulated through radial canals into the circular canal.
8. Excretion: There are no special organs for excretion. Nitrogenous wastes diffuse out
into the water through external as well as internal surfaces. As there is only one opening
i.e. mouth in both polyp and medusa, wastes released inside are thrown out of the mouth.
9. Respiration: Exchange of gases takes place through general surface of the body as the
colony is present within water. As water also enters through mouth in the gastrodermal
cavity and is circulated throughout the colony, each cell is in direct contact with gases
dissolved in water. Oxygen diffuses inside and the carbon dioxide diffuses out in the
water which moves out of the colony through mouth.
10. Reproduction:
A. Asexual mode of reproduction: Obelia colony grows in size by budding of new
horizontal stems (hydrorhiza) and vertical stems (hydrocauli), thereby increasing the
number of zooids. Blastostyles form medusae (reproductive zooids) by budding.
B. Sexual mode of reproduction: Medusa is a free swimming reproductive zooid
which is formed by budding from the blastostyle. Medusa, once sexually mature is set
free and released out through gonopore of gonangium into the water. A medusa is
dioecious i.e. it bears either male or female gonads. There is no sexual dimorphism (male
medusa and female medusa cannot be identified externally). Each medusa bears four
gonads at the per-radial position on the subumbrellar surface. Each gonad has an outer
ectoderm and inner gastrodermis. Ectoderm of the gonad is in continuation with the
ectoderm of the subumbrellar surface while gastrodermis of the gonad is in continuation
with the gastrodermis of the radial canals. The germ cells do not arise in the gonads but
they develop from the interstitial cells of the ectoderm of the blastostyle at the time of
budding of medusa. Later on these germ cells migrate through radial canals and take
their position within the gonads. Gonads either give rise to sperms or ova. Mature
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sperms are shed into the sea water by the rupture of gonads. Ova may be either released
in the water or they may remain within the gonads to be fertilized. Medusa dies once it
releases sex cells in the water.
a. Fertilization: Fertilization may take place in the water if both ova and sperms are
released in the water or sperms may fertilize the ova within the gonads of female medusa.
b. Life cycle: A single sperm fertilizes one egg to form the zygote. Zygote undergoes
complete (holoblastic) and equal cleavage to form a solid ball of cells, called morula. It
develops into another stage called blastula with a cavity within called blastocoel. Two
layered gastrula is formed by a process of delamination (from the inner surface of the
blastomeres are cut off new cells which fill up the blastocoel at one pole). Thus the
hollow blastula changes into a two layered gastrula with an outer ectoderm and inner
endoderm.
i. Planula larva: Gastrula becomes elongated and its outer ectoderm gets uniformly
ciliated (Fig. 8), it has at first an inner solid core of endodermal cells which eventually
split to develop a cavity called coelenteron. This larva has a distinct broad anterior end
and a narrow posterior end. Its cells get differentiated into columnar ectodermal, sensory,
gland, nerve cells and nematocysts. Nerve cells and sensory cells are present in great
number at the anterior end. As planula larva is ciliated, it leads a free living life for
sometime but it cannot feed on its own as it has no mouth. After a short span of free
living, it settles down, attaches to the substratum which can be a stone, log of wood, or
any solid object. After settling down, it undergoes metamorphosis and changes to next
larval stage called hydrula stage (polypoid form).
ii. Hydrula larva: Its attached end forms the basal disc while its free end develops a
mouth on a manubrium with number of tentacles surrounding it. This larva resembles a
miniature Hydra and thus called a hydrula larva (Fig. 9). Hydrula grows further by
extensive process of budding of hydrocaulous and hydrorhiza and is soon converted into
a complex branching colony of Obelia like parental colony.
While free swimming medusa in the life cycle of a fixed Obelia colony helps in the
dispersal of gametes and it is free swimming planula larva which disburses the species to
newer locations. This doubly ensures that species do not over crowd in one area.
11. Alternation of generation and Metagenesis: In Obelia, two types of individuals,
the fixed polyp and the free swimming medusa alternate successively so that polypoid
generation asexually gives rise to medusae, which reproduces sexually to produce
zygotes developing later on into polypoid form of larva. This hydrula larva develops into
a new colony by asexual budding (Fig. 10). But it is not true alternation of generation as
reported in plants which are discussed below.
A. Alternation of generation: True alternation of generations is better explained in a
plant (fern). Here the fern is diploid and known as sporophyte. It reproduces asexually
producing haploid spores. Each spore develops in a haploid generation called
gametophyte. The gametophyte produces haploid ova and spermatozoa which after
fertilization, give rise to the diploid generation called sporophyte. So, the life cycle of a
fern shows alternation of an asexual, diploid, sporophytic generation with the sexual,
haploid gametophytic generation. This is known as true alteration of generation.
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B. Metagenesis: In Obelia, both colonial, hydroid phase and free living medusoid
phases are diploid. Medusa represents diploid phase because it originates from the
diploid blastostyle by budding. The sexual generation is represented only by the haploid
gametes which are in fact derived from the epidermis of the blastostyle ad then migrated
to gonads of medusa. So medusa is not producing the gametes, but is meant only for
dispersal of gametes. Therefore, in Obelia it is diploid hydroid colony which alternates
with the free swimming diploid medusa. This type of alternation of generation is called
metagenesis. Obelia thus does not show the true alternation of asexual phase with the
sexual phase as mentioned in fern earlier. In brief, metagenesis is the regular alternation
of the colonial, asexual, diploid, polypoid generation with the solitary, free moving,
diploid, sexual, medusoid generation.
12. Polymorphism. : It is the phenomenon of occurrence of several structurally and
functionally different individuals within a species. Any species having different
individuals performing different functions for its survival and dispersal is called
polymorphic. It results in the formation of a colony bearing different individuals with
division of labor. Colony remains fixed to the substratum or may be free living with
certain different modifications. Obelia is one of the very important organisms which
exhibit the phenomenon of polymorphism as it bears three distinct types of zooids.
A. Polyps: These are feeding zooids or gastrozooids, and are meant for the feeding of
the whole colony. Food captured by gastrozooids is shared with other zooids via the
common coelenteron present within the colony.
B. Blastostyles: These are asexual polyps and produce medusae by budding from them.
Once the medusae are fully formed, they are released out of them through gonopore.
C. Medusae: These are diploid reproductive zooids and reproduce sexually by producing
haploid sperms or ova in the water. They lead a free living life and help in dispersal of
gametes and prevent overcrowding of the species.
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Tentacles
extended
Polyp
Tentacles
contracted
Hydrotheca
Blastostyle
Polyp retracted
Gonopore
Gonotheca
Gonangium
Medusa bud
Perisarcal annuli
Perisarc
Hydrocaulus
Coenosarc
Hydrorhiza
Fig. 1: External features of an Obelia colony.
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Mouth
Manubrium
Nematocyst
Tentacle
Epidermis
Mesogloea
Endoderm
Hydrotheca
Shelf
Coenosarc
Perisarc
Coenosarcal
canal
Fig. 2. Longitudinal section of a polyp.
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Gonotheca
Medusa
.
Gonopore
Gonangium
3a
Gonopore
Gonotheca
Medusa
Blastostyle
Coenosarcal canal
Annuli
3b
Fig. 3a. A Gonangium arising from the colony of Obelia.
3b. A gonangium showing blastostyle and budding medusa.
Page 28 of 115
Exumbrellar surface
Gonad
Velum
Exumbrellar surface
Tentacles
Epidermis
Subumbrellar
surface
3c
Gastrodermis
Mesogloea
Radial canal
Gastrodermal cavity
Stomach
Circular canal
Tentacle
Velum
Manubrium
Mouth
Gonad
Subumbrellar surface
Per-radial
tentacle
Inter-radial
tentacle
Circular canal
Ad-radial
tentacle
Location of
statocyst
Manubrium
Mouth
Radial canal
3d
Gonad
Fig. 3c: Lateral view of a medusa. Fig. 3d: Subumbrellar view of medusa (oral view)
Fig. 3e. Lateral view of medusa showing positioning of gonads and other structures.
Page 29 of 115
3e
Gastrodermis
Epidermis
Gland cell
Sensory cell
Interstitial cells
Epithelio-muscular cell
Endothelio- gland
cell
Cnidoblast
Germ cell
Interstitial cells
Sensory cell
Nutritive
cell
Nerve cells
Mesogloea
Fig. 4: L.S. of Body wall of Hydra.
Page 30 of 115
Discharged
nematocyst
Epidermis
Fig. 5a: Part of the epidermis showing cnidoblasts.
Page 31 of 115
Cnidocil
Operculum
Nematocyst
Cnidoblast
Coiled thread
Nucleus
5b
Barbules
Barb
Operculum
Nematocyst
Everted thread
Muscular fibers
5c
Fig. 5b: Undischarged nematocyst. Fig. 5c. Discharged nematocyst.
Page 32 of 115
Penetrant
Volvent
Glutinant
Undischarged
Nematocysts
Discharged
Nematocysts
Fig.6a: Types of nematocysts
Page 33 of 115
Penetrant nematocyst
Enlarged view of the tail region of this larva
Penetrant
Volvent
Fig. 6b. An insect larva paralyzed by penetrant nematocysts and volvents.
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Circular canal
Ectoderm
Gastroderm
Mesogloea
Interstitial cells
Tentacular base
Outer nerve ring
7a
Inner nerve ring
Statocyst
Sensory cells with processes
Otolith
Lithocyte with a nucleus
7b
Fluid filled vesicle
Fig. 7a: Statocyst at the base of an adradial canal in L.S.
7b. Structure of a single statocyst in a section.
Page 35 of 115
8
Ciliated
ectoderm
Coelenteron
Endoderm
Tentacles
Polyp
Annuli
Substratum
9
Fig.8: Planula Larva shown in a section.
Fig. 9. Hydrula larva
Page 36 of 115
Female medusa (2n)
Obelia colony
(2n)
Male medusa (2n)
Ovum (n)
Sperm (n)
Zygote (2n)
2-cell
stage
Hydrula larva (2n)
Life cycle of Obelia
Blastula (2n)
Young polyp
Gastrula forms by
delamination
Longitudinal section
(L.S.)
Ciliated Planula
Fig. 10. Life cycle of Obelia
Page 37 of 115
IV. Aurelia aurita (Jelly-fish)
1. Introduction: The class scyphozoa is represented primarily by the medusoid form,
which is more dominant phase in their life history whereas the polypoid stage is restricted
only to the larval stage which is known as scyphistoma. Aurelia aurita is a typical
example of Class - Scyphozoa (which means ‘cup like animals’) and is commonly
known as jelly fish or sea jelly. There are around 200 species of jelly fishes and all of
them are found in marine water.
The name jelly fish or sea jelly is assigned to Aurelia aurita because its body is
transparent due to jelly like massive mesogloea present between epidermis and
endodermis. It does not have any resemblance to fish (which is a highly evolved
vertebrate) except that it is a water living animal. There is also no relation between two
as both of them have entirely different structures.
2. Habit and Habitat:
Aurelia aurita is marine water living animal and is the inhabitant of coastal waters in
warm and temperate seas. It may remain solitary (live alone) or in groups floating quietly
by rhythmic contractions. They are thrown by sea currents on the sea shore during
storms.
3. Structure:
Aurelia is a medusoid form; however it differs from the medusa of Obelia in many
respects (Table 1). For instance oral arms while present in Aurelia, are absent in a typical
hydrozoan medusa. Sensory organs of Obelia are more advanced (rhopalium) than
statocyst of hydrozoan medusa.
Aurelia has a transparent body and has a radial symmetry (Fig. 1a). It has an inverted
cup like body which is gelatinous in nature. Size may vary from 7 cm. to about 60 cm.
however; some are quite large and may be 2 meter across. It has slightly convex
exumbrellar surface and concave subumbrellar surface and its body can easily be
identified by the presence of four horse-shoe shaped gonads in the centre of the
subumbrellar surface a little away from the manubrium. The body parts can be shown
with reference to particular radii of the bell or cup.
A. Oral arms and Mouth: A very short manubrium is present in the centre of the
subumbrellar surface which bears a mouth at its tip (Fig. 1b). There are four frilled and
delicate oral arms arising from the four angles of the mouth. The four arms are almost as
long as the radius of the bell and their broad bases are united to surround the mouth.
Each oral arm bears longitudinal, ciliated grooves leading into the mouth. The thin edges
of the grooves are convoluted, and bear numerous short tentacles (fimbriae) with batteries
of nematocysts, but the base is thick. Cross section of an arm is ‘V’ shaped.
B. Radial canals: The corners of the mouth and the four radial canals arising from them
are per-radial in position (Fig. 1b and Fig. 2). Midway between four per-radial canals are
4 inter-radial canals. Per-radial and inter-radial canals are branched and rebranched
before joining the circular canal. Further, there is one ad-radial canal present in between
Page 38 of 115
each per-radial and inter-radial canal giving rise to total 8 ad-radial canals. The body is
roughly circular in outline interrupted with eight notches placed at the 4 per-radial and 4
inter-radial positions. Each notch hosts a sensory organ called tentaculocyst or rhopalia
protected by a pair of small lobes called marginal lappets.
C. Velarium: There is a thin, delicate, flexible, marginal flap called velarium which is a
continuation of the sub-umbrellar surface and bears tentacles and 8 sensory organs called
rhopalia all along the margin. It has gastrodermal canals running through it. It is
different from the velum of the medusa of Obelia because velum is only a narrow fold of
subumbrellar ectoderm and there are no canals running through it in contrast to velarium.
D. Sub-genital pits: Sub-genital pits are four in number and lies just beneath the gonads
and are inter-radial in their location. These are small rounded apertures which lead into
a small shallow cavity. They do not have any special function in relation to gonads.
E. Gonads: There are four gonads. Each gonad is present just above the sub genital
pits at the inter-radial position. They are horseshoe-shaped structures and are visible
through jelly like body.
4. Histological structure: The arrangement of body wall layers and cell types are
similar to that of Obelia or Hydra.
A. Epidermis: The exumbrellar and subumbrellar surfaces, the tentacles, velarium, oral
arms and manubrium are covered externally by epidermis (Fig. 3). The gullet and the
sub-genital pits are also lined by invaginated epidermis. Exumbrellar surface bears
epithelial cells while sub-umbrellar surface has epithelio-muscular cells in addition to
nerve, sensory, gland and stinging cells (cnidoblasts). Epithelio-muscular cells of the
subumbrellar surface are striated while muscles of the tentacles, manubrium and the oral
arms are simple longitudinal muscles.
B. Mesogloea: It is sandwiched between epidermis and gastrodermis. It is highly
gelatinous and forms a very thick sheet on the exumbrellar surface. It contains branching
fibers and amoeboid cells derived from the epidermis.
C. Gastrodermis: The gastrovascular cavity (stomach), radial canals originating from
stomach and their extensions into the rhopalia or tentaculocysts, circular canal at the
periphery etc. are all lined internally by the gastrodermis. Gastrodermis consists of
flagellated endothelial cells, gland cells and nerve cells.
5. Nervous System: Nervous system of Aurelia is more advanced than that of medusa
of Obelia. It consists of (A) main nerve plexus present on the subumbrellar surface, and
(B) a diffuse nerve net present both on exumbrellar and subumbrellar surfaces.
A.
Main nerve plexus: Main nerve plexus constitutes a rapid conducting system of
stimulus through bipolar cells. It is present below the ectoderm on the subumbrellar
surface and helps in coordination of muscular activity during locomotion. It is also
extended in the oral arms, manubrium, tentacles and rhopalia. It is made up of bipolar
nerve cells and nerve fibers arising from them forming a continuous net work. These
nerve cells form special thickening at the base of tentacles present at the per-radial and
inter-radial positions. This ganglion like concentrations of nerve cells coordinates with
the rhopalia present near them. Sensory impulses received by the sense organs are
Page 39 of 115
conveyed to the muscle fibers and mesogloea to control the movements of the jelly fish
by them.
B. The diffuse nerve net: The diffuse nerve net constitutes a slow conducting
system of multipolar neurons. It is present in ectoderm of both exumbrellar and subumbrellar surfaces. The nerve cells are smaller with multipolar nerve fibers and help in
recognition of local stimulus and animal can respond accordingly.
6. Sense organs:
Aurelia aurita freely swims in the water showing periodic
movements from surface waters to deeper waters and back again. They also form
temporary breeding aggregations. At times they are drifted upside down with the water
currents. To maintain themselves in the right orientation, they have developed special
sensory organs like rhopalium for maintaining equilibrium, ocelli-to respond to light and
olfactory pits- sensitive to smell. All these structures coordinate with each other to give
a proper response.
A.
Rhopalia (Tentaculocysts): They are modified tentacles and hence called
tentaculocysts. There are eight characteristic sensory organs called rhopalia. Out of
which four are located at per-radial and four are at inter-radial position. Each rhopalia is
located between a notches formed by a pair of specialized tentacles called marginal
lappets and is protected over by hood like process of bell margin (Fig. 4a, 4b). Each
rhopalium consists of a hollow club shaped projection enclosing an extension of circular
canal lined by gastrodermis. At the apical end of gastrodermal cells, a spherical mass of
cells called statolith is present (Fig. 4b). Its function is just like the statocyst of the
Obelia. Statolith is made up of calcium sulphate and calcium phosphate particles. There
is a region of ciliated sensory cells (adoral olfactory pit explained a little later) in the
epithelial layer just beneath the tentaculocyst. If the animal is tilted, the tentaculocyst
containing statocyst press against the ciliated sensory cells present below, with the result
a stimulus is generated in the associated nerve cells of main nerve plexus present on the
subumbrellar surface. Tentaculocyst makes the animal aware of its displaced orientation
and coordinates the muscular activity and the animal comes to its original position. So
rhopalium act in coordination with the sensory cells present nearby and maintains the
equilibrium of the jelly fish by controlling the rhythmic action of the swimming bell.
B. Ocelli: Two types of ocelli are present in Aurelia. One is pigment spot ocellus and
the other is pigment cup ocellus (Fig. 4b).
a. Pigment spot ocellus is exposed on the outer side of the epidermal cells of the
tentaculocyst. It consists of photoreceptor cells packed with light sensitive pigment and
is connected with the nerve net present underneath.
b. Pigment cup ocellus is situated on the inner side of the tentaculocyst in association
with the statocyst. It is in the form of an inverted cup made by pigmented and sensory
gastrodermal cells. The pigment cup ocellus is connected with the underneath nerve net.
Both these ocelli (Pigment spot ocellus and Pigment cup ocellus) are simply light
receptors which help the animal to come to the surface or go down in the water according
to the availability of light. Both these ocelli are non image forming light receptors.
C. Olfactory pits: Olfactory pits are the regions lined with sensory cells present in the
epidermis. There are two olfactory pits, outer or aboral olfactory pit is present on the
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exumbrellar surface at the base of the hood, and the inner or adoral olfactory pit is
present below the tentaculocyst.
7. Locomotion: Locomotion in Aurelia is brought about by the sudden rhythmic
contractions of the muscles and gelatinous mesogloea. Mesogloea functions as a skeletal
system relaxing the muscles after their contraction. When the muscle fibers of the jelly
fish contract, the volume of the fluid under the bell decreases, with the result water is
forcibly expelled from under the bell and the animal is propelled in the opposite
direction. Due to contraction of muscles, the gelatinous mesogloea is displaced. When
the muscles relax, mesogloea comes back to its original form and the volume of water
enclosed by the swimming bell increases and the jellyfish is pushed downward.
8. Food and feeding: Aurelia is carnivorous and its food consists of planktons, eggs,
and larval stages of many marine invertebrates. Sometimes they also derive nutrition
from zooxanthellae (unicellular algae) that live in symbiotic relationship with them.
Feeding in Aurelia may takes place (i) by using oral arms and tentacles: as the jelly fish
swims and the prey comes in contact with the tentacles or oral arms, both these
structures contract and bring the prey near the manubrium and then taken within the
mouth for further digestion or (ii) they may behave as suspension feeders by trapping
planktons with the help of mucus on the subumbrellar surface. The mucus laden food is
then swept towards the bell margin, from where it carried by flagellated grooves within
the oral arms towards the mouth and then to the stomach.
9. Digestive system: Digestive system here is better known as a system of fluid-filled
gastrovascular canals. Gastrovascular canals help in the circulation of oxygen and carbon
dioxide (vascular part), as well as in the circulation of food (gastro part) and hence called
gastrovascular system. Gastrovascular system consists of mouth, gullet, stomach, gastrogenital canal, gastric pouches, adradial canals, circular canal, per-radial and inter-radial
canals, exhalent canal, gastro-oral canal, basal groove of oral tentacle.
Nematocysts present on the oral arms or tentacles paralyze the food particles and carry
into the mouth. Food is then carried to four gastric pouches bearing gastric filaments (Fig.
2 and 3). Gastric filaments bearing nematocysts paralyze or kill and digest the prey with
the help of digestive enzymes they secrete. Partly digested food is then conveyed to the
stomach. Gland cells present within the gastrodermal cells release enzymes for
extracellular digestion while intracellular digestion takes place by phagocytosis of the
food particles within the gastrodermal cells. Digested food is then circulated through
radial canals by the ciliated gastrodermal cells, while undigested food is thrown out of the
mouth by exhalent current of water. The path of food circulation is explained in the
water circulation.
10. Water circulation: Circulation of water plays an important role in the distribution of
food and oxygen in Aurelia. Inhalent current of water enters the mouth, passes through
the narrow gullet into the stomach and then to gastric pouches. Each gastric pouch
contains an opening to help circulation of the water. From gastric pouches, water enters
through the unbranched ad-radial canals and reaches the circular canal (Fig. 2). The
exhalent water current returns by the branched inter-radial and per-radial canals back to
gastric pouches. Per-radial canals bring water current to gastro-oral canal and then to the
stomach, while inter-radial canals bring water to exhalent canal, to gastro-oral canal and
Page 41 of 115
then to stomach. From the stomach water goes out through basal groove tract in the oral
arms.
11. Respiration and Excretion: The highly branched gastrovascular cavity and
constant circulation of water brings in oxygen along with food while carbon dioxide and
excretory wastes (ammonia) are carried away from the body either by outgoing current
of water or diffuses out through general surface of the body.
12. Reproduction: Asexual mode of reproduction is completely absent in Aurelia.
Sexual reproduction takes place by sex cells produced in the gonads. Sexes are separate
i.e. sperms and ova are produced by different individuals. Sexes cannot be differentiated
externally (no sexual dimorphism). There are four horse-shoe shaped gonads in each
sex, which are present at the per-radial position (Fig. 1b). Gonads develop within
gastrodermal tissue unlike Obelia where gonads develop from the ectodermal tissue.
Gonads are closely associated with the gastric pouches and their (gonads) concavities
face towards the stomach. Mature sperms from male Aurelia are released from gonads
into the stomach and passed out through mouth by outgoing current of water. While a
mature ovum is discharged from the ovary, get fertilized by incoming sperm inside the
gastric pouch (internal fertilization).
13. Life Cycle: After fertilization, zygote reaches the oral groove through water
current. Oral groove acts as the temporary brood chamber as further development takes
place there (Fig. 9). By repeated divisions, zygote develops into a ball of cells called
morula. Morula changes into a single layered blastula with a cavity inside called
blastocoel. Blastula undergoes a process of invagination and a two layered gastrula is
formed. Gastrula has an outer ectoderm and inner endoderm. The gastroderm lines a
central cavity called coelenteron having a small opening to outside called blastopore
which is not completely closed. It is in contrast to Obelia where gastrulation takes place
by delamination. Gastrula elongates, its outer ectoderm develops cilia and blastopore
gets closed. Larva at this stage is named as planula larva (Fig. 5a, 5b). These larvae can
be seen lodged on the oral arms of the female Aurelia.
Planula larva: Once Planula larva is fully developed, it escapes out of the oral arms and
freely swims in the water with the help of cilia. Planula larva cannot feed on its own as
it does not have any mouth. Therefore, after swimming for sometimes it settles down
and attaches itself to substrate, loses its cilia and a mouth is being developed at the
opposite end of attachment. Soon tentacles are developed around the mouth. Its attached
end develops into a basal disc. In this way planula larva is being transformed into a
small Hydra like polypoid larval stage called hydratuba or young scyphistoma larva
(Fig.6a).
B. Scyphistoma larva: It is the only polypoid stage in the life cycle of Aurelia. It is
attached to the substratum. Scyphistoma larva is sessile (fixed) like Obelia colony. It has
no statocyst and ocelli like adult Aurelia. Its attached end elongates and narrows down to
a basal disc. Its free end develops a squarish mouth and its edges elongate to form
manubrium. Firstly, four per-radial tentacles bud out around mouth. Subsequently, other
four inter-radial and then eight ad-radial tentacles are developed. So, larva has developed
sixteen tentacles around its mouth (Fig. 7). It starts feeding on its own. Transverse
section of a scyphistoma larva shows the presence of a diploblastic body wall (Fig. 6b).
The gastrodermis lining the coelenteron is raised into four per-radial gastric pouches
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alternating with four gastric ridges or taenioles. These gastric pouches and gastric ridges
partially divide the gastrovascular cavity. Each gastric ridge bears rudimentary gonads,
septal muscles and a sub-umbrellar funnel in the centre. All four gastric ridges bear small
gastric filaments hanging towards the gastrovascular cavity. The gastric ridges are
present only in the larval stage of Aurelia and are absent in the adult. Presence of gastric
ridges is one of the characteristic features of the scyphozoans and these are considered as
preliminary evolution of mesenteries present in anthozoans.
As scyphistoma larva feeds on its own, it grows in size. It either undergoes a process of
strobilation or it may multiply asexually by process of budding and can form new
hydratuba which separates from the parent attaches to the substratum and grows into
another scyphistoma.
C. Strobilation: Scyphistoma undergoes a process of strobilation. During strobilation,
larva stops feeding. Strobilation is a process during which the body of scyphistoma
subdivides transversely forming number of saucer shaped structures piled one above the
other. At this stage it is known as a strobila. Each saucer shaped structure is known as
ephyra larva. Ephyrae break away from the strobila one by one as they grew up and start
swimming freely in the water. First ephyra larva which breaks from the parent bears the
original tentacles of the scyphistoma, which falls later on. When many ephyrae are
produced from single scyphistoma, it is called polydisc strobilation. Polydisc strobilation
takes place when food is in plenty and the temperature is low. When the food is in
scarcity and the temperature is high, only one ephyra is produced from the scyphistoma
larva, it is called monodisc strobilation.
D. Ephyra Larva: It is a young, medusoid form larva of Aurelia. It has an exumbrellar
and subumbrellar surface. Its body has eight bifid arms; out of which four are per-radial
and four are inter-radial (Fig. 8a and 8b). Each arm is notched at the terminal end to
form a pair of marginal lappets. In between the marginal lappets, there is a short tentacle
which later on develops into rhopalium or tentaculocyst. All oral arms are free from each
other at the periphery and joined in the form of a disc like structure towards the centre.
At this stage, 8 rayed ephyra acquires a star like structure. In between each arm a
rudimentary bud of ad-radial canal can be seen. The ephyra contains a small segment of
stomach of scyphistoma with gastric ridges. On the exumbrellar surface, stomach is
closed, while on the subumbrellar side it is open. Its edges grow out to form a short
manubrium bearing a squarish mouth at its terminal end. Cavity of the stomach grows
into eight arms forming eight radial canals. Gastric ridges are replaced by full fledged
gastric filaments.
E. Metamorphosis: The ephyra freely swims in the water feeding on microorganisms.
As it grows, mesogloea thickens enormously, so that two gastrodermal layers come
together forming a solid lamella between the gastrodermal radial canals. The ad-radial
buds grow rapidly and fill the gaps between per-radial and inter-radial arms. All sixteen
radial canals are fully formed and all of them finally open in the circular canal which is
developed at the periphery of the bell. Now, star shaped, 8 rayed ephyra becomes
circular and saucer-shaped like the adult medusa (Fig. 9). Further, four oral arms around
mouth and numerous marginal tentacles appear at the periphery of the medusa. A young
medusa changes its physical appearance altogether and becomes an adult Aurelia.
Page 43 of 115
14. Alternation of Generation:
Aurelia reproduces sexually whereas the hydratuba and scyphistoma reproduce asexually.
Aurelia is medusoid in nature and the hydratuba and scyphistoma are polypoid forms as
they resemble a typical polyp structure. Therefore, it can be said that the medusoid phase
alternates with the polypoid phase.
But, Aurelia does not present a true case of
alternation of generation, because here the medusoid phase develops as a result of
metamorphosis of an ephyra which is developed as one of the several segments of
scyphistoma. The life cycle represents metamorphosis complicated by asexual
multiplication in the larval condition.
Table 1. Comparison of Aurelia and medusa of Obelia colony.
S. No.
Aurelia ( a medusoid form)
Medusa of Obelia colony
1.
2.
Saucer shaped with exumbrellar and
subumbrellar surfaces.
Bell margin is notched into 8 lobes bearing 8
rhopalia present at the per-radial and inter
radial positions.
3.
Rhopalium endodermal
Bell shaped but grows from
blastostyle.
Bell margin is smooth and
bears 8 sense organs
(statocysts) present at the
base of the ad radial
tentacles.
Statocyst ectodermal
4.
4 unbranched radial canals
are present.
No ocellus present.
6.
8 branched and 8 unbranched radial canals
are present.
Pigment cup ocellus and pigment spot ocelli
are present.
Velarium is present
7.
4 oral arms are present around the mouth.
Oral arms absent.
8.
Absent
10.
Gastric ridges bearing gastric filaments
present.
4 gonads present internally at the base of
gastric pouches
4 Sub genital pits are present.
12.
Fertilization internal
External fertilization
13.
Gastrulation by invagination.
By delamination
14.
Planula → Scyphistoma→Ephyra→adult
Aurelia
Planula→Hydrula→
Obelia colony
5.
9.
Page 44 of 115
Velum is present
4 gonads hang externally
from the radial canals.
Absent.
Exumbrellar surface
Gonad
Rhopalium
Marginal Tentacles
Oral arms
Subumbrellar surface
Fig. 1a. External features of Aurelia as seen from the exumbrellar surface.
Page 45 of 115
Tentacles
Velarium
Gastric
filaments
Gonad
Branched Interradial canal
Sub-genital
pit
Mouth
Branched perradial canal
Unbranched adradial canal
Gastric pouch
Rhopalium
Fig.1b Subumbrellar view of Aurelia.
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Oral arms
Mouth region
Gastro-genital duct
Gastro-oral
canal
Gastric pouch
Exhalent canal
Gonad
Per-radial canal
Ad-radial canal
Inter-radial canal
Circulatory
canal
Location of Rhopalia
Fig. 2. Diagrammatic representation of the gastrovascular system in Aurelia. A part of subumbrellar view
is enlarged. Arrows represent the circulation of water current.
Page 47 of 115
Exumbrellar surface
Gastric filament
Stomach
Gonad
Inter-radial canal
Mesogloea
Ectoderm
Gastroderm
Circular canal
Hood
Sub-genital pit
Radial canal
Gullet
Mouth
Subumbrellar surface
Fig. 3. Diagrammatic vertical section of Aurelia showing gonads and gastric filaments.
4a
Page 48 of 115
Hood
Olfactory pit
Statocyst
Tentacles
Marginal lappets
Outer olfactory pit
Gastrodermis
Hood
Pigment spot ocellus
Extension of circular
canal
Pigment cup
ocellus
Mesogloea
Statoliths
Amoebocyte
Nerve cells
Epidermis
Ciliated sensory cells
Inner olfactory pit
4b
Figs. 4a. Location of rhopalium in relation to tentacles and marginal lappets and
4b. Vertical section of rhopalium.
Page 49 of 115
Blastopore not
fully closed
Ciliated
ectoderm
Mesogloea
Gastroderm
Enteron
Ciliated
ectoderm
5a
5b
Fig. 5a. Planula larva.
5b.Vertical section of planula larva.
Page 50 of 115
Tentacles
Septal funnel pit
Mouth
Bud
Basal disc
6a
Septal muscles
Gastrodermal cavity
Epidermis
Mesogloea
Gonad
Gastrodermis
Gastric pouch
Gastric
filaments
Subumbrellar
funnel
Gastric ridge
6b
Fig. 6a. A young scyphistoma larva.
6b. Transverse section of a scyphistoma larva showing gastric ridges and other structures.
Page 51 of 115
Tentacles
Mouth
Mature Ephyrae
Young ephyrae
produced by transverse
fission
Stalk
Basal disc
Fig. 7. Strobilating scyphistoma larva.
Page 52 of 115
Exumbrellar
surface
Arms
Mouth
Subumbrellar surface
8a
Primary Lappets
Buds of adradial canal
Mouth
Gastric ridges
Per-radial
canal
Rhopalium
Inter-radial
canal
8b
Fig. 8a. Ephyra larva (lateral view).
8b. Ephyra larva (subumbrellar view)
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Adult male
Adult female Aurelia in a section
Gonad
Ephyra larva
Fertilization
Oral arm
Sperm enters female medusa
Life cycle of Aurelia
Zygote
2-celled stage
Blastula
Gastrula forms
by invagination
Ciliated planula larva
Scyphistoma
larva
Young
strobila
Planula larva
(A section)
Fig. 9. Life cycle of Aurelia aurita.
Page 54 of 115
V Polymorphism
1. Introduction: The presence of polymorphism in cnidarians is one of their
characteristic features. It is defined as the occurrence of structurally and functionally
different types of individuals within the same organism during its life cycle. A species
that exhibits this phenomenon is called polymorphic. Polymorphism is predominantly
exhibited by the different animals of class- hydrozoa. Hydroid colonies which bear two
types of zooids are known as dimorphic, while colonies which bear more than two types
of zooids are called polymorphic colonies. There are different types of zooids which
have evolved in different animals according to their habit and habitat and are discussed in
this section.
2. Class- Hydrozoa: Phenomenon of polymorphism is exhibited generally by the
animals of class hydrozoa but a few animals belonging to class anthozoa like Penaatula
(sea pen) also show dimorphism. Degree of polymorphism can be explained from the
following examples.
A. Order- Hydroida:
a. Hydra: Hydra is the fresh water animal in contrast to majority of cnidarians discussed
below which are marine water animals. It remains singly and is attached to the weeds by
its base with the mouth hanging down. Hydra is the simplest type of cnidarians having
only polyp form as it has no trace of medusa either in adult form or in larval stage. As it
exists only in one form and all the functions are performed by the polyp itself, it is
considered as the monomorphic form. It has a cylindrical structure of which the lower
end is attached to the substratum while other free end always hangs down and bears a
small mouth located on the raised structure called manubrium. Mouth is surrounded by
number of filiform (pointed) tentacles bearing batteries of nematocysts. Mouth leads into
a central body cavity called coelenteron. It reproduces asexually by producing buds and
sexually by producing sex organs developed from the ectoderm of the body (Fig. 1a, b)
b. Obelia: It is an important example of the trimorphic colony. As already discussed
earlier in detail, it has two basic forms of zooids, i.e. polyp and medusa. Polyp and
medusa are fundamentally similar and can be derived from each other hypothetically.
Although, both of them are different in structure and function (Fig. 2) the polyp form
exists in 2 structurally and functionally different forms making the total number of zooids
to 3 types.
i. Polyp: It is sessile (fixed to the substratum) with a hydra like body attached to the main
colony by narrower end. Its free end is wider and raised into hypostome that bears a
mouth surrounded by tentacles. It faces upwards and carries the function of feeding the
colony. Polyps are specialized for feeding and thus known as gastrozooids (Fig. 2).
ii. Blastostyle: Blastostyle is club shaped and it arises as an extension of the main colony
from the axils of the branches and produces medusa by process of budding. It is
protected by a thin transparent perisarc called gonotheca having an opening called
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gonopore. Blastostyle cannot feed on its own. Medusa when mature can come out of the
gonopore to swim freely in the water.
iii. Medusa: It is a free swimming zooid with an umbrella shaped body having
exumbrellar and subumbrellar surfaces. It has a mouth facing downwards (present on the
tubular growth called manubrium hanging down from subumbrellar surface) in contrast
to polyp in which mouth faces upwards. Like hydranth, it can feed on its own for its
survival until its function is over. It bears four gonads (testis or ovary) which produces
either sperms or ova at the time of maturity and is responsible for sexual reproduction
therefore also known as gonozooid. They normally die after reproducing the colony.
Therefore, they not only help in sexual reproduction but also play an important role in
dispersal of the colony.
c. Bougainvillea: It is a colonial, fixed, dimorphic hydrozoan. It has creeping stolons
called hydrorhiza attached to the substratum from where vertical branches arise. Each
vertical branch produces polyps and medusae. Each polyp is present at the terminal end
of the branch and has a conical manubrium bearing a mouth, surrounded by two circlets
of tentacles. A thick, chitinous, perisarc covers hydrorhiza and vertical branches arising
from it. There is no hydrotheca as is present in the Obelia. Inside the perisarc is present
coenosarc which is made up of ectoderm, mesogloea and gastroderm. Small rounded
buds arise from the coenosarc of the branches (Figs. 3a, b). Each bud develops into a
medusa. Developed medusa has a bell like structure bearing gonads for the sexual
reproduction. It breaks from the stalk at the time of maturity and swims freely. Medusae
do not arise from blastostyles as they arise in Obelia.
d. Tubularia: It may occur singly or in colonies fixed to the substratum in marine water.
It is dimorphic as it bears two types of zooids- gastrozooid and gonophores present very
close to each other. Gastrozooid is flower like, has two circlets of tentacles. Oral
tentacles surrounding mouth are smaller while tentacles present near the base are longer.
Reproductive zooids are called gonophores as they are degenerate medusae which hang
down in the form of clusters from the base of the gastrozooid (Fig. 4). They bear both
male and female medusae, so colony is dioecious. The medusae are never set free.
Fertilization takes place within female gonophore. Zygote develops into planula larva
which remains fixed (sessile) in the gonophore which further develops into another larval
stage called actinula larva which gets free from the gonophore and freely swims in the
water before it settles to metamorphose into adult Tubularia. So, Tubularia has free
swimming actinula larva in contrast to many other coelenterates where planula larva is a
free swimming larva.
e. Hydractinia: It is a small, colonial, marine and highly polymorphic animal present in
shallow waters on rocks, gastropodan shells inhabited by hermit crabs etc. When
Hydractinia is found attached to the gastropodan shell occupied by hermit crab, it does
not have a symbiotic relationship with the crab as it is capable of managing its food on its
own but it gets an extra advantage of being mobile along with the hermit crab. Hermit
crab also gets an advantage of being protected as predators will not harm it because of the
nematocysts loaded on the tentacles of the Hydractinia zooids. There is an epizoic
(external benefit) relationship between both of them.
Hydractinia has five different types of zooids which are directly attached to the
hydrorhiza which forms a brownish mat over the substratum (Fig. 5).
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i.
Gastrozooids: These are feeding zooids. They have a large size with a mouth
and one circlet of long, capitate (knobbed) tentacles at the base of conical
manubrium.
ii.
Gonozooids: These are reproductive zooids which are without mouth and
tentacles. They are smaller than gastrozooids. They are degenerate medusa and
bear a group of sac like gonophores containing eggs or sperms. They have a
short, rounded hypostome which bears knob-like projections bearing
nematocysts.
iii.
Spiral dactylozooids: These zooids are without mouth and tentacles. As their
name suggests they are spirally coiled, more elongated and bear numerous
nematocysts at their terminal ends. They provide protection to the colony by
having stinging cells (nematocysts).
iv.
Tentaculozooids: These are long blunt ended zooids which are present at the
periphery of the colony. They are devoid of mouth and tentacles but have
clubbed knobs bearing nematocysts.
v.
Skeletozooids: Many thin, long, spine like structures are also present which
provides support and thus forms the skeleton of the colony.
f. Vellela: It has an elliptical, umbrella like body resembling a typical medusa bearing a
vertical crest like structure called sail on the dorsal surface. There is a pneumatophore on
the dorsal side having chitinous, concentric chambers containing air and opening through
fine pores on the dorsal side. Around the margin, it bears simple tentacles called
dactylozooids (protective). A big single gastrozooid with a big mouth is present in the
centre on the sub-umbrellar surface (Fig. 6). Gastrozooid is surrounded by numerous
gonozooids, each having a mouth at its tip. Gonozooids bud off free swimming medusae
at the time of maturity. There are ectodermal canals originating from the sub-umbrellar
surface and opening into the pneumatophore chambers. Gastrozooids and gonozooids
open inside the gastrodermal canals, which further open into the gastrodermal cavity
present inside the body. It is photosymbiotic and harbor symbiotic algae in their polyps
or medusa.
g. Porpita: Its structure resembles Vellela, except a sail. It has a disc like body
enclosing a chambered chitinous shell containing air in it (Figs. 7a, b and c). It has a
large central gastrozooid, surrounded by 3-4 circlets of gonozooids which are further
surrounded by dactylozooids bearing nematocysts, arranged at the periphery of the disc.
Medusae are budded off from gonozooids. It is also photosymbiotic like Vellela.
B. Modifications of polyp: Polyps structurally get modified into different types of
zooids according to the requirement of an individual, which are described below:
i.
Gastrozooids: These are feeding zooids and resemble the structure of polyp
without usual tentacles. They are tubular, elongated, with a mouth facing
towards the bottom of the colony. From the base of the polyp arise one or two
long, contractile, hollow tentacles which bear lateral contractile knobbed
branches (tentilla) at regular intervals. These knobs are the batteries of
nematocysts (Fig. 8a and b).
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ii.
Dactylozooids: These are protective zooids. They may be also called as feelers
or palpons. Structurally, they look like gastrozooids but are blind structures
without any mouth. They may have a long tentacle arising from the base but
unlike tentacles of gastrozooid, do not bear branches (Fig. 8a, 8b).
iii.
Gonozooids: They may resemble gastrozooids having mouth but are without
tentacles (Fig. 8a, 8b) and bear medusa. In other the gonozooids may form
stalked branches bearing grape like structures called gonophores. Sometimes
tentacles like dactylozooids are attached to them which are called gonopalpons.
iv.
Pneumatophore: It is a hydrostatic apparatus present in siphonophores. It is gas
filled chamber that appear to be a highly modified polyps (although previously
considered as derived from medusae). The pneumatophore is the first zooid
formed from the planula larva in those animals that possess it. It helps in
keeping the body in an upright condition while floating. It is without mesogloea
but the umbrella cavity contains an air chamber called a pneumatocyst (Fig.
8a). Cells lining the pneumatocyst secrete the gases or may expel out of it
through one or more small openings called stigmata. Thus pneumatophore is a
balloon like structure or a hydrostatic chamber containing air. It is present in
those forms which have a long stem and helps in keeping the body in an upright
position (Fig. 9, 10a and 11a). When pneumatophore is filled with air, the
colony becomes lighter and floats at the surface of the water, but when the gas
is expelled out of the pneumatophore, colony sinks down.
C. Modifications of medusa: Medusoid zooids are being modified into much
different type of structures which help the colony to swim (Fig. 8a and b). These
may be of following types :
i.
Nectocalyx (nectocalyces-plural): These are small swimming bells,
resembling medusa in having radial canals, circular canal and velum with
a deeply concave muscular subumbrellar surface. They do not bear mouth,
tentacles and sense organs, probably because their main function is to
make the colony lighter, so that these colonies can swim freely and easily
(Fig. 8a). They move the colony through water by their contractions.
ii.
Nectophore: It is another kind of modified medusa which is present in
Diphyes. It is a large conical swimming bell having a groove into which
whole colony can be retracted inside (Fig. 10a). It helps in swimming of
the colony as Diphyes does not have any separate pneumatophore for
swimming.
iii.
Hydrophyllum or bract: Hydrophyllum as its name suggests, is a small
leaf like, protective structures modified from umbrellas of medusa but
having a reduced gastrovascular cavity and have a cleft on one side. In
many forms, these are much bigger in size. Bract on the other hand, is a
small leaflet and is much smaller than the hydrophyllum. Bracts are
attached to each cormidium (Fig. 10b, 11b). They partially cover the
cormidium and help them in protection and in floating.
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All above mentioned different type of zooids are originated as buds from the main axis of
the stem and are lined by ectoderm and endoderm. Their cavities communicate with the
cavity of the stem.
D. Order- Siphonophora: This order exhibit highest degree of polymorphism. It
includes complex, floating, polymorphic colonies produced by budding from an
originally medusoid or polypoid form. Here, Polyps and medusae are modified into
different zooids which remain attached to the same colony and perform specific
functions. They do not possess any sensory organ found in other floating organisms like
Aurelia and medusa of Obelia. The characteristic feature of this order is the presence of
swimming bells, nectocalyces or a pneumatophore (an air filled balloon like structure)
meant for the floating of the colony. The structure of a generalized siphonophore
exhibits different types of modifications of polyp and medusa (Fig. 8a). Examples of few
animals are discussed below to explain polymorphism.
a. Physalia: It is commonly called as Portuguese man-of- war because it can suddenly
appears and disappears from the surface of water by increasing or decreasing the gas
content within its pneumatophore. It possesses a large pneumatophore (a modified polyp
lying horizontally) on the dorsal side filled with air to float in the water. There is a small
opening at the front end of the pneumatophore which leads into a chamber called
pneumatocyst (Fig. 9), the air secreting cells are present at the base of the pneumatocyst.
The main component of the gas within the float is around 90 percent carbon monoxide
and rest is oxygen. The float has a crest formed by a fold of the trunk on the dorsal side.
The pneumatocyst extends into the crest and is divided by a number of transverse septa
into air chambers. A group of zooids arise from the ventral side of the float in a multiple
series. Each group of zooid arising from the same stalk is called a cormidium (pluralcormidia). Each cormidium bears short and long dactylozooids (without mouth),
gastrozooids (with mouth), and clustered gonozooids. Long dactylozooids sometimes
may reach a length of 6 feet and have especially large stinging cells (nematocysts) that
can paralyze the prey immediately and may cause burning sensation, followed by fatal
injury even to humans.
b. Diphyes: It consists of a long contractile hollow stem bearing at the apex two opposed
nectocalyces or swimming bells without manubrium, but with four radial canals, a
circular canal, and velum. At regular intervals a series cormidia present. At the point
where the stem joins two nectocalyces, there is a deep groove called the hydroecium, into
which the contractile stem with its cormidia can be retracted. In the jelly of the upper
most nectocalyx is a space lined by large vacuolated cells, and sometimes containing an
oil drop. This is a dilatation of the upper end of the central canal of the stem and is called
the stomatocyst. The buds of the cormidia are always formed at the upper end of the
stem, so that the oldest cormidium is the lowest. Cormidia are separated by equal sized
internodes, thus called endoxiform (Fig. 10a and 10b). Each cormidium consists of two
medusoid individuals-the one of these is a sterile and the other is a fertile medusoid form.
The sterile medusoid consists of a bract or hydrophyllum, a siphon (trumpet shaped
polyp), and a tentacle, while the fertile one is a gonophore. The siphon is the manubrium
of the sterile medusoid, which is displaced from its umbrella and has a trumpet-shaped
mouth at its free end. The tentacle is the marginal tentacle of the medusoid which has
shifted on to the base of the manubrium. The tentacle is tubular and is beset with a series
of lateral tubular tentilla. The gonophore has a 4-radiate canal system and a velum but
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without tentacles and mouth.
manubrium.
The sex cells originate from the ectoderm of its
c. Helistemma: It shows bilateral symmetry in contrast to other coelenterates which
show radial symmetry. It is a colonial animal showing a higher degree of polymorphism.
It has a long stem bearing a single float (pneumatophore) without a terminal opening of
the neumatocyst (an air chamber) on its dorsal side.
A number of swimming bells
(nectocalyces) are arranged on the both sides of the stem (Fig. 11a and 11b). Each bell
has four radial canals, circular canal, a velum and sometimes ocelli. This part of the stem
helps in floating of the colony and thus called nectosome. Below the swimming bells,
rest of the colony, bearing a number of cormidia arranged in groups, is called
siphonosome. Each cormidium consists of one siphon (gastrozooid) with tentacle,
mouthless dactylozooid with a tentacle, several bracts (hydrophyllia), two types of
gonozooids (male and female) and a palpon like structure which has a terminal opening
(anal like) which help in excreting fluid and wastes. The batteries of nematocysts are
present on the tentilla attached to the tentacles. Female gonozooid produces only one egg
which after fertilization gives rise to a zygote, which develops into planula larva. One
end of the planula larva develops a pneumatophore by invagination while the other end
forms a polyp which elongates and develops into an elongated colony by asexual
budding.
3. Class Anthozoa:
Polymorphism is generally absent from anthozoans but one
example is given below which shows dimorphism.
a. Pennatula (Sea Pen): Its structure is like a quill feather. It has a lower peduncle and a
distal rachis (stem). They are found in the warmer coastal waters mainly where bottoms
are soft, so that their peduncle can easily be buried in the sea bottoms. There is a horny
axial skeleton in the stem and calcareous spicules in the mesogloea. It is anchored in the
sand by its peduncle. It is dimorphic and bears two types of zooids (Fig. 12a). From the
both sides of the rachis arise lateral branches bearing feeding zooids called autozooids or
anthocodia (Fig. 12b). These zooids are joined at their bases in one plane and are
attached to the main rachis. They are feeding zooids. On the back side of the rachis are
present, small polyps called siphonozooids which have no tentacles and helps in the
circulation of water in the colony.
From the account given above it is clear that coelenterates exhibit polymorphism and
that polymorphism has evolved mainly in the colonial animals and that too in the
hydrozoans except few anthozoans. The importance of polymorphism is reflected to
have division of labor among different forms and help in the survival and dispersal of the
colonial animals. In addition it helps in avoiding overcrowding of species at a particular
area.
Page 60 of 115
Twig
Pedal disc
a.
Tentacles
Mouth
Testis
Body
Bud
Ovaries
b.
Fig. 1a. Hydra hanging down from the twig.
1b. Hydra.
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Blastostyle
Medusa budding from
blastostyle
Polyp
Fig. 2. An Obelia colony showing trimorphism (presence of three types of zooids).
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Hydranth/ Polyp
a
Contracted hydranth
Medusa buds
Exumbrellar surface
Oral tentacle
Fig. 3a. Bougainvillea colony (a part of the colony).
Sense organ
Radial canals
Marginal tentacles
.
b. Single medusa of Bougainvillea
Page 63 of 115
Gastrozooid
Oral tentacles
Aboral tentacles
Stomach region
Gonophores
Stalk
Fig. 4. Tubularia hydranth.
Page 64 of 115
Nematocysts
Gastrozooid
Tentaculozooid
Nematocysts
Spiral dactylozooid
Young dactylozooid
Female gonozooid
Male gonozooid
Substratum
Dactylozooid
Fig. 5. Hydractinia showing different types of zooids.
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Sail or crest
Pneumatophore with
Concentric air chambers
Edge
Gastrovascular cavity
Tracheal canal
Gonozooids with medusae
Dactylozooids
Mouth
Gastrozooid
Fig. 6: Diagrammatic section of a Vellela showing different types of zooids.
7a.
7b.
Page 66 of 115
Pneumatophore
Gonozooids
Dactylozooids
Dactylozooids
Gastrozooid
7c.
Chambered Pneumatophore
Gastrozooid
Medusa
Gonozooid
Dactylozooids
Fig. 7a. Porpita, dorsal view, b. subumbrellar view. c. A section of Porpita.
Page 67 of 115
Pneumatophore
Ectoderm
Nectocalyces
Endoderm
Hydrophyllium
Enteron
Gonophore
Tentacle
Dactylozooid
Tentilla
Tentacle of
dactylozooid
Gastrozooid
Fig. 8a. A generalized diagram of a siphonophore.
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Battery of
nematocysts
Gastrozooid
Dactylozooid
Tentacle
Basal tentacle
Nematocyst knobs
Tentilla
Gonophore
Gonopalpon
Gonozooids
Testis
Ovary
Male gonophore
Female gonophore
Fig. 8b: Modifications of polyp and medusa.
Page 69 of 115
Sail
Gas chamber
(Nematocyst)
Gastrodermal cavity
Short
dactylozooid
Cormidia
Gonozooids
Gastrozooid
Long
dactylozooid
Mouth
A single cormidium
Fig. 9: Part of a Physalia showing the structure of a cormidium.
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Cavity of
swimming
bell
Swimming bells
(Nectophores)
Coenosarc
Group of zooids
(Cormidium)
Bract
Medusa
Gastrozooid
Tentacles
10 a.
10 b.
Fig.10a: Diagrammatic view of Diphyes colony.
Fig. 10b. Enlarged view of a Cormidium (group of zooids).
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Pneumatophore
Swimming bells
or Nectocalyces
Nectosome
Tentacle
Coenosarc
Tentilla
Siphonosome
Gastrozooid
Gonozooids
Hydrophyllum
(Bract)
Dactylozooid
Unbranched tentacle
a
b (a cormidium)
Fig. 11a: A colony of Helistemma.
11b. A cormidium.
Page 72 of 115
Anthocodia / Autozooids
Pinnulae bearing
anthocodia
Rachis
Anthocodia
Pinnulae
Peduncle
Rachis
Siphonozooids
12a
12b
Fig. 12a. Pennatula colony.
12b. A part of the Pennatula showing two types of zooids.
Page 73 of 115
VI Mesenteries
1. Introduction: Mesenteries are present only in class anthozoa. All anthozoans are
polypoid animals and live in marine water. Mesenteries are septa which grow from the
gastrodermis of the column body wall and join the pharynx in the upper half of the body.
They thus divide the coelenteron into many chambers up to the pharyngeal region.
Before discussing the variations of mesenteries in different animals, it is of great
importance to study the basic structure and mesenteries of a typical anthozoan- the
Metridium.
2. Structure of Metridium (Sea Anemone) explaining Mesenteries:
The structure of Sea Anemone (Metridium) is represented here as it is a typical example
of the class anthozoa. Longitudinal section of sea anemone (Fig. 1) shows the
positioning of mesenteries and other associated structures present within the body. It has
a polyp like structure which can be divided into three regions, oral end, column and basal
disc. Oral end bears a mouth surrounded by a circlet of tentacles. Tentacles are hollow
and the gastrovascular cavity is extended into them. Mouth is a slit like opening guarded
by circular muscles. It leads into a pharynx which is lined from inside by an ectoderm,
mesogloea and gastroderm. There are two ciliated grooves called siphonoglyphs which
allow circulation of water within gastrovascular cavity even if mouth is closed. Pharynx
opens into the gastrovascular cavity or the coelenteron. Column represents the main
body of the animal. Basal disc is used for the attachment of the animal to the substratum.
The gastrovascular cavity is divided by paired mesenteries which according to their size
and position are differentiated into primary, secondary and tertiary mesenteries.
A. Primary mesenteries or complete mesenteries: These are those paired mesenteries
that are connected to the body wall on one side and to the stomodaeum
(pharynx/siphonoglyph) on the other side. These completely divide the gastrovascular
cavity into separate chambers. This is shown in a transverse section of Metridium
through stomodaeum (Fig. 2a). Below stomodaeum, primary mesenteries hang in the
coelenteron (Fig. 2b). Each mesentery is lined on both sides by the gastrodermis with a
thin sheet of mesogloea in between as these are the extensions of the gastrodermis
towards the pharynx.
The part of the gastrovascular cavity or coelenteron enclosed between a pair of
mesentery is called endocoel while within the adjacent pairs is called exocoel. The
mesenteries of each pair bear longitudinally arranged parietal and retractor muscles.
There is a specific arrangement of retractor muscles on the mesenteries. Retractor
muscles are present on one side of each mesentery and are visible as bulging present on
the mesenteries. These retractors of adjacent mesenteries either face towards each other
(facing endocoel thus called endocoelic) or do not face each other (facing exocoel thus
exocelic). Generally, mesenteries attached to the siphonoglyph have retractors facing
away from each other and thus are called directives.
B. Secondary mesenteries or incomplete mesenteries: These are those mesenteries
that are connected to the body wall and do not reach to the stomodaeum (Fig. 2b), they
hang freely in the coelenteron or gastrovascular cavity. They may be secondary or
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tertiary mesenteries. Six pairs of secondary mesenteries are present between the primary
mesenteries and do not reach the gullet, thus hang halfway from the body wall into the
gastrovascular cavity. While, 12 pairs of tertiary mesenteries are much smaller and are
present in exocoels present between the primary and secondary mesenteries. Secondary
and tertiary mesenteries are formed later than the primary mesenteries during
development. So in Metridium there are total 24 pairs of mesenteries including 6
primaries, 6 secondaries and 12 tertiaries. The free ends of incomplete mesenteries are
trilobed in cross section (Fig. 3a), having two lateral lobes and one central lobe (Fig. 3b).
Lateral lobes are lined by ciliated endothelial lining and called ciliated lobes while central
lobe bears lot of nematocysts and glandular cells producing enzymes and mucous, thus
called cnido-glandular lobe. Lower down in the coelenteron, ciliated lobes disappear and
only cnidoglandular lobe remains and forms the absorptive part of the gastrodermis and
bears batteries of nematocysts to paralyze the prey (Fig. 3c). In some sea anemones, the
ends of the mesenteries are drawn into thread like structures called acontia, containing
mucus cells and cnidoblasts. These acontia can come out of the small openings called
cinclides present in the column body wall (Fig. 1). Acontia provides protection and help
in the digestion of food as they bear number of nematocysts (stinging cells) and
enzymatic cells within in their gastrodermal lining. Near the terminal ends of the
incomplete mesenteries, gonads are present which bear sex cells for the reproduction.
They are dioecious and sperms and ova are released in the water through mouth and
fertilization takes place in the water. Zygote develops into a free swimming ciliated
planula larva which attaches to the substratum and grows into an adult sea anemone.
There is an extensive asexual reproduction by budding, or by splitting of the polyp into
parts, where each part develops into a new polyp or by pedal laceration in which animal
moves further and a part of the pedal disc is left behind. Detached pedal disc regenerates
into another sea anemone.
The number and arrangement of mesenteries is different in different animals of class
anthozoa which is obvious by looking at the following examples:
3. Octocorallians: are polypoid animals which are colonial, bear eight pinnate feather
like tentacles, 8 complete mesenteries, have one siphonoglyph and supporting skeleton is
endoskeleton which is soft or horny, perforated by gastrodermal tubes that are continuous
with the gastrovascular cavity of the polyp and are supported by calcareous spicules.
4. Hexacorallians: are polypoid animals which are colonial bear either six pairs of
mesenteries or in the multiples of six. The number of mesenteries is generally equal to
the number of tentacles. In hexacorallians, the six largest tentacles correspond to the six
primary mesenteries and are developed first during the course of development. Later on
more tentacles are added which corresponds to the secondary mesenteries and so on.
5. Examples of anthozoans showing different arrangement of mesenteries:
a. Alcyonium: It has all the diagnostic features of octocorallians. The polyps
(anthocodia) have 8 pinnate tentacles and 8 mesenteries (Fig. 4). The siphonoglyph is
single and the side on which it is present is called ventral (no homology to the ventral
surface of higher animals). A pair of mesenteries attached to the siphonoglyph have
longitudinal muscles facing towards the endocoel ( also called sulcul mesenteries) while a
pair of mesenteries present on the opposite side (i.e. dorsal side in respect to
siphonoglyph) bear muscles facing away from each other(towards exocoel and are called
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asulcal mesenteries). The dorsal mesenteries are often longer than the others, and are
developed earlier in the bud but later in the egg. Rests of the four mesenteries are
unpaired and bears muscles which are facing down (Fig. 5a and b). Below pharynx, six
of the mesenteries hang freely in the coelenteron and bear unilobed ends while asulcal
pair of mesenteries bear cilia and help in circulation of water within a polyp and also
throughout the colony for the renewal of food and oxygen. Alcyonium has one of the
simplest body forms and simplest type of arrangement of mesenteries found in the
anthozoans. Similar type of arrangement of mesenteries is also present in the polyps of
Tubipora. Many of the hexacorallians like sea anemone and stony corals pass through a
larval stage (Edwardsia stage, Fig. 6) in their development in which they resemble a
young alcyonarian polyp in having 8 mesenteries.
b. Edwardsia: It has also total 8 mesenteries like Alcyonium. The Tentacles are more in
number as compared to the number of mesenteries. There are two siphonoglyphs, so two
pairs of mesenteries are directives and are attached to both the siphonoglyphs. Muscles
present on the both pairs of directives do not face each other in contrast to Alcyonium
where muscles face each other on one pair of directives (Fig. 6). Rests of the four
mesenteries are unpaired and their muscles face downwards like Alcyonium. Edwardsia
is an important animal as it resembles the octocorallian structure (have 8 mesenteries)
and helps in tracing the origin of hexacorallians from octocorallians.
c. Gonactinia: The arrangement of mesenteries of this anthozoan resembles the
arrangement seen in Edwardsia. There is an addition of few mesenteries in this animal
e.g. 4 new mesenteries (microsepta) facing 4 already existing unpaired mesenteries (seen
in Edwardsia) have started growing (Fig. 7). Another difference is that two pairs of
secondary mesenteries (incomplete) have also grown up. With the result this animal has
total 8 pairs of mesenteries, out of which two pairs are secondary mesenteries, two pairs
are directives and 4 pairs are under completion.
d. Halcampoides: It is a primitive sea anemone having two siphonoglyphs. There are
only 6 pairs (12) of primary mesenteries out of which two pairs are directives bearing
exocoelic muscles (Fig. 8) and four pairs are bearing endocoelic muscles. These first
formed paired and complete mesenteries are called protocnemes. Secondary and tertiary
mesenteries were not developed at all in this animal. .
e. Halcampa: It has six pairs of primary mesenteries like Halcampoides. In addition it
has 6 pairs of small underdeveloped 6 pairs of secondary mesenteries called microsepta
in the region of stomodaeum (Fig. 9). So, in this animal rudiments of secondary
mesenteries are reported which are completely formed and are reported in the Adamsia.
f. Adamsia: It has two siphonoglyphs. There are total six pairs of primary mesenteries
out of which two pairs are called directives and the longitudinal muscles present on them
do not face each other while longitudinal muscles present on rest of the four pairs of
mesenteries face each other. 6 pairs of secondary mesenteries are fully developed each
having endocoelic retractor muscles (Fig. 10). Even, 6 pairs of tertiary mesenteries are
also reported in this animal. Now, if we compare the mesenteries of Adamsia with the
Metridium, both of them have similar type of arrangement of mesenteries (Fig. 2a).
g. Haloclava: It is decamerous as it has total 10 pairs of primary/complete mesenteries
some of which are formed by the extensions of the micro-septa joining the stomodaeum
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(Fig. 11). Both pairs of directives have muscles towards the exocoel while rest of the
mesenteries has retractor muscles facing towards the endocoel.
h. Zoanthus: The arrangement of mesenteries is quite different from others and bears
either 6 or in the multiples of six. There are total 12 mesenteries out of which 6 are
complete mesenteries and 6 are incomplete mesenteries. Out of 6 complete mesenteries,
two directives are complete and two directives are incomplete (Fig. 12). Rest of the 4
pairs of mesenteries consists of one complete (macro-septa) and other incomplete (microsepta) mesenteries with endocoelic retractor muscles in each pair. Such an arrangement
of mesenteries is called brachycnemous.
i. Epizoanthus: It has total six pairs of mesenteries out of which, 3 pairs are complete
mesenteries (macrosepta) i.e. the ventral directives, 4th and 5th mesenteries on each side
starting from the dorsal directives (Fig. 13), while 3 pairs are incomplete mesenteries(1st,
2nd and 3rd pair).
j. Cerianthus (a burrowing anemone): Sand living, elongate, solitary polyps. They
secrete a tube of the discharged threads of ptychocysts (special cnidoblasts). Cerianthus
has a single siphonoglyph and many complete, unpaired and coupled mesenteries. Their
number is not fixed, as it grows, more and more mesenteries are added towards the dorsal
growth zone (dorsal interseptal space) and not between already existing couples (Fig. 14).
The two ventral directives attached to siphonoglyph are small. The mesenteries present
on either side of the directives are larger and reaches to the aboral end. The mesenteries
decrease in size towards the dorsal region where new mesenteries are added. The
retractor muscles are absent in all the mesenteries.
k. Antipathes (Black coral): It has six tentacles. Mesenteries may be 4, 6 or 10. There
are 10 complete mesenteries having rudimentary retractor muscles (Fig. 15a). Another
section shows presence of four directive mesenteries and other two lateral mesenteries
which bear gonads for the sexual reproduction. In addition there are 4 secondary
mesenteries (Fig. 15b). Mesenteries are without mesenteric muscles. Skeleton is
branched, made up of scleroprotein (antipathin), consists of black ectodermal chitinous
axis covered with thorns. It is found in deep waters.
l. Peachia: It burrows in the sand when adult but its larval form live as parasites or
commensal in the radial canals of scyphomedusae (Fig. 16). It has 6 pairs of complete
mesenteries and 4 pairs of incomplete mesenteries.
m. Metridium: As we have seen in fig.2a where it has 6 pairs of complete mesenteries,
6 pairs of secondary mesenteries and 12 pairs of mesenteries which are developed
sequentially giving rise to total 24 or 12 pairs of mesenteries.
6. Formation of mesenteries: Formation of mesenteries have been reported in some
larval forms of hexacorallians i.e. Actinaria. Mesenteries are not added at random but
they are formed in a specific manner. Some of the stages have a particular arrangement
of mesenteries which are found to be present in some adult anthozoans. This is evident
from the arrangement of mesenteries discussed earlier in various anthozoans.. The
formation of mesenteries is explained below in brief (Fig 17a to 17f).
Step 1: Initially considering there is no mesentery within the coelenteron. First of all
two mesenteries (1st pair mesenteries) are formed at right angles to the pharynx and
divide the gastrovascular cavity into two chambers; one is dorsal and bigger (LC) while
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the other is ventral and smaller chambers (SC) giving rise to bilateral symmetry (Fig.
17a).
Step 2: 2nd pair mesenteries started appearing in the bigger chamber (LC) Fig. 17b.
Step 3: 3rd pair of mesenteries begin to appear within the smaller chamber (SC) shown in
Fig. 17c.
Step 4: 4th pair of mesenteries are formed within the chamber enclosed by 2nd pair (LC)
mesenteries, just opposite to the 3rd pair of mesenteries (Fig. 17d). This type of
arrangement is found to be existed in Edwardsia (Fig. 6).
Step 5: Two paired mesenteries grew between the 1st, 2nd and 1st, 3rd. Newly formed
mesenteries remain incomplete and their longitudinal muscles face already existing 2nd
and 1st pair mesenteries (Fig. 17e). In this way, 8 complete and 4 incomplete mesenteries
are formed which are also found to be present within Gonactinia (Fig. 7) and with little
modification in Zoanthus (total 12 complete or incomplete mesenteries (Fig. 12).
Step 6: A pair of small mesenteries destined to be incomplete mesenteries start appearing
within exocoels (Fig. 17f). In Peachia only four pairs of incomplete mesenteries are
found to be present. Later on, more and more mesenteries are added which may be
secondary or tertiary mesenteries as are already discussed.
In the examples discussed above, 3rd and 4th pair of mesenteries became the two pairs of
directives. It is very clear that secondary and tertiary mesenteries are developed only in
the exocoels. The arrangement of longitudinal muscles is also very specific as the
muscles present on the directives faced exocoelic chambers in majority of animals, in
contrast to other mesenteries where muscles faced endocoelic. The gonads are developed
in the mesenteries and the sex cells are lodged in the endoderm. Once the sex cells are
mature, they are released in to the coelenteron.
7. Significance and function of mesenteries: The main function of the mesenteries is
to increase the surface area. The free terminal ends of the mesenteries are very long and
bear more number of enzymatic cells, mucous cells and cnidoblasts. Their increased
surface area not only helps in killing the prey but also helps in extracellular and
intracellular digestion. Retractor muscles present on the mesenteries also help in
retraction and invagination of the oral disc and tentacles. If a sea anemone is over
stimulated, it retracts its body fully and acontia are protruded out of the mouth or through
cinclides in the column. Here, the nematocysts present on the acontia are used for the
defense purpose and for killing the prey. Besides they also help in excretion and
production of gonads for the sexual reproduction.
The tentacles are also developed in a similar order to that of the developing mesenteries.
First pair of tentacles appears with the appearance of the dorsal bigger chamber (LC) and
is longer than rest of the tentacles which are added later on as the embryo grows into an
adult. From the edges of the mouth arise siphonoglyphs which help in maintaining a
continuous supply of water inside the gastrovascular cavity even if the mouth is closed.
The number, type and arrangement of mesenteries determine the classification pattern of
most of the anthozoans. In addition they also help in tracing the evolution of many
anthozoans. Radial symmetry in anthozoans is also lost due to the presence of
siphonoglyph and mesenteries. With the result, bilateral symmetry (when one
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siphonoglyph is present), or biradial symmetry (when two siphonoglyphs are present) is
attained. Bilateral symmetry or biradial symmetry is attained during embryonic
development and persists into the adult which is another sign of gradual evolution of
higher animals having bilateral or biradial symmetry.
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Mouth
Oral disc
Tentacles
Fossa
Siphonoglyph
Oral ostium
Capitulum
Parapet
Sphincter
Longitudinal
retractor muscles
Pharynx
Marginal ostium
Complete mesentery
Mesenteric
filament
Transverse muscle
Mesenterial filaments
Gonads
Gastrovascular
cavity
Acontium
Limbus
Cinclide
Parietal muscle
Epidermis
Acontia
Basal disc
Fig. 1: Longitudinal section of Metridium showing mesenteries and other structures.
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Exocoel
Endocoel
Paired secondary
mesenteries
Epidermis
A pair of Primary
mesenteries
Gastrodermis
Directive
couple
A pair of Tertiary
mesenteries
Pharynx
Siphonoglyph
Gonads
Retractor muscles
.
Gastrodermis
2a
Epidermis
Exocoel
Mesogloea
Endocoel
Mesenteric filaments
Coelenteron
2b
Fig.2a. Diagrammatic transverse section of Metridium through pharynx showing mesenteries.
2b. Diagrammatic transverse section of Metridium below pharynx showing hanging mesenteries.
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Epidermis
Mesogloea
Gastrodermis
Parietal
muscles
Retractor muscles
Gonads
Cnidoglandular lobe
3a.
Fig. 3b
Fig. 3c
Mesogloea
Ciliated cells
Ciliated tract
Gland cells
A core of
mesogloea
Gland cells
Nematocyst
Nematocyst
Cnidoglandular lobe
Cnidoglandular lobe
Fig.3a: T.S. of the body wall of sea anemone showing trilobed cnidoglandular lobe of a single mesentery.
3b. Magnified view of T. S. of a trilobed end of mesentery (upper part). 3c. Magnified view of T.S. of
unilobed end of mesentery (lower part).
Page 82 of 115
Pinnules
Tentacles
Mouth
Opening of pinnule
Siphonoglyph
Mesenteries
Coelenteron
Solenia
Soft skeleton
due to
gastrodermal
tubes
Gastrodermal
tubes
Axial skeleton
.
Fig. 4: Structure of an octocorallian polyp
Page 83 of 115
Asulcal mesenteries or directives
Asulcal mesenteries having ciliated ends
Retractor
muscles
facing
outwards
Endocoel
Coelenteron
Exocoel
Epidermis
Mesogloea
Pharynx
Siphonoglyph
Stomodaeum
Sulcal mesenteries or directives
With retractor muscles facing inwards
5a
Gastrodermis
Unilobed end of mesentery
5b
Asulcal mesenteries or directives,
Retractor muscles facing outwards
Exocoel
Secondary
mesenteries
Endocoel
Primary
mesenteries
Microsepta
Siphonoglyphs
Directives retractor
muscles facing outwards
6
7
Fig. 5a: T.S. Alcyonium through pharynx showing arrangement of mesenteries.
5b: T.S. Alcyonium below pharynx.
Fig.6. Cross section through Edwardsia.
Fig. 7. Cross section through Gonactinia.
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A pair of Primary
mesenteries
Exocoelic retractor
muscles
Exocoel
Endocoel
Siphonoglyphs
Endocoelic
retractor
muscles
Fig. 8. Cross-section through pharynx of Halcampoides showing 6 pairs of complete mesenteries.
Siphonoglyphs
Secondary
mesenteries
Endocoel
Exocoel
A pair of Primary
mesenteries
Fig. 9. Cross section through pharynx of Halcampa showing 6 pairs of complete (primary mesenteries
while 6 pairs of secondary mesenteries are beginning to appear.
Page 85 of 115
Exocoel
A pair of
primary
mesenteries
Secondary
mesenteries
Endocoels
Tertiary
mesenteries
Fig. 10. Cross section through pharynx of Adamsia.
Retractor muscles
Siphonoglyphs
Endocoel
Exocoels
Fig. 11. Cross section through pharynx of Haloclava showing 10 pairs of primary mesenteries.
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Incomplete
Dorsal directives
Complete
mesenteries
Incomplete
mesenteries
Complete ventral directives
Fig. 12. Section of Zoanthus through pharynx showing 6 complete and 6 incomplete mesenteries.
Incomplete
Dorsal directives
1
1
2
2
Incomplete
mesenteries
3
3
4
4
5
5
6
6
Complete ventral directives
Fig. 13. Section of Epizoanthus through pharynx.
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Complete
mesenteries
Ventral directives
Siphonoglyph
All Complete
mesenteries
Ventral exocoel,
new mesenteries are
added here.
Youngest mesentery
Fig. 14. Section through pharynx of Cerianthus.
Siphonoglyph
Incomplete
mesentery
Siphonoglyphs
Poorly
developed
retractor
muscles
15b
15a
Complete mesenteries
Fig. 15a. Antipathes section with 10 complete mesenteries.
Fig.15b. Antipathes section showing 6 complete mesenteries and 4 incomplete mesenteries.
Page 88 of 115
Secondary
mesenteries
Siphonoglyph
Primary
mesenteries
Fig. 16. Section of Peachia through pharynx.
Page 89 of 115
a
b
2
LC
2
LC
Dorsal Larger
chamber (LC)
LC
LC
1
1
SC
1
1 Ventral Smaller
SC
chamber (SC)
4
c
d
2
2
2
2
LC
LC
1
LC
LC
1
SC
1
BC
LC
SC
1
SC
Present in
Edwardsia
3
3
e
f
4
4
2
2
5
5
Incomplete
mesenteries
1
1
6
Present in
Gonactinia
and Zoanthus
6
3
6
6
Present in Peachia
3
Figs. 17a to 17f. Formation of mesenteries. They are numbered according to the order of their
development.
Page 90 of 115
VII Corals and coral reefs
1. Introduction:
Corals are hard structures that are made up of calcium carbonate and are deposited into
big stony formations by the anthozoans which are exclusively marine water living. All
anthozoans are polypoid animals without any medusoid form at any stage of the
development. The polyps produce sex cells, fertilization gives rise to formation of zygote
developing into planula larva, which directly form new polyp. These anthozoans may
remain singly or form big colonies and majority of them secrete hard calcareous
structures around them starting from their basal disc. Anthozoans bear cilia on different
regions of the body e.g. the cilia present around the mouth beat towards outer side and
thus clean the oral end.
Colonial and fixed polyps show a great degree of development by secreting enormous
structures called coral reefs which play an important role in maintaining aquatic
ecosystem. Coral reefs are submarine ecosystems which can be compared with the rain
forests as they also host diverse flora and fauna. The reef building corals require warm
shallow waters i.e. above 200C. They are therefore limited to the Indo-Pacific, Central
Western Pacific, and the Caribbean regions north to Bermuda, while other corals live at
moderate depths throughout the world.
In addition to different types of the coral forming anthozoans, there are variety of other
organisms belonging to different groups which also contribute significantly in the
formation of big stony structures over the years called coral reefs. Different anthozoans
and hydrozoans which contribute to the formation of coral reefs are briefly discussed
below.
2. Class: Anthozoa: A majority of the coral forming animals belongs to class anthozoa
and structurally resembles sea anemones except that they do not have siphonoglyph. A
coral polyp resembles the basic structure of the sea anemone.
A. Subclass: Octocorallia or Alcyonacea
a. Structure of Octocorallian coral:
Octocorallians are mostly represented by the Alcyonarians (soft corals) and gorgonians in
many coral reefs. Besides this, the red coral (Tubipora) and Blue coral (Heliopora)
having calcareous, massive skeleton are also coral forming octocorallians and contribute
in the formation of coral reefs. Each Octocorallian bears 8 pinnate tentacles (tentacles
have small side branches like a feather), and have 8 complete mesenteries on either side
of the base of the tentacle. One siphonoglyph is present. Polyps are small and are joined
to each other by the skeleton called coenenchyma which is secreted by the ameobocytes
present within the mesogloea (Fig. 1a). Therefore, the skeleton of the octocorallians is
internal in contrast to external skeleton present in the hexacorallia. The skeleton may
consist of spicules of different sizes and shapes (Fig. 1b) or horny strands made up of
calcium carbonate. Coenenchyma consists of thick mesogloea which is perforated by the
gastrodermal tubes which are in continuity with the gastrovascular cavities of the polyp
bodies. Besides these gastrodermal tubes there is a network of small ciliated
interconnecting tubes called solenia which help in maintaining a current of water inside
the colony. It is covered on the outer side by the epidermis which is continuous with the
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epidermis of the polyp. Lower part of the polyp is present within the coenenchyma so
that only upper part pf polyp is protruded out of it. They are commonly found in warm
waters of Indo-Pacific Ocean. A few examples are given below to have an idea about
different types of corals.
i. Alcyonium (Dead man’s finger or soft coral): It is the soft coral and is one of the
simplest body forms of the anthozoans. It bears only feeding polyps. The polyps are
embedded in a fleshy mass called coenchyma from which only delicate oral ends of the
polyps (anthocodia) protrude and can be withdrawn within the coenenchyma at the time
of danger (Fig. 2). Polyps are located only on the free end, whereas the proximal part is
sessile (fixed) is devoid of polyps. Skeleton is made up of separate calcareous spicules
which form a mass of calcium carbonate after their death.
ii. Heliopora (blue coral): It is bright blue in color and is found living along with the
true corals on the coral reefs of the Indo-Pacific. They live in symbiotic relationship with
zooxanthellae so are zooxanthellate. It is the only octocorallian in which the secreted
spicules are not separate but form a massive calcareous skeleton perforated by numerous
closely set cylindrical cavities (diverticula’s) closed below (Fig. 3) which increase the
surface area. Skeleton is lobed and bears polyps which can withdraw themselves within
the pores. Live polyps are always present on the surface of the skeleton as colony grows.
The skeleton formed by one polyp is called a corallite while many polyps form a big
coral structure which is known as corallum. The corallum not only have pores for the
polyps but also has tubular cavities of intermediate size which sometimes are occupied by
a chaetopod worm, (Leucodore genus). When worms are protruded out from their holes
in search of food, polyps get retracted within their pores.
iii. Tubipora (Organ pipe coral): Tubipora as its name suggests has long, horny, tubes
joined by horizontal platform in which polyps are present. The base of the polyp grows
out and form horizontal platform containing a tube inside called solenia from where new
polyps arise (Fig. 4). Polyps lie parallel to each other and their internal skeleton is made
up of fused spicules produced by the ameobocytes present within mesogloea. With the
result large colonies of the coral are produced. Polyps are present in the tubes partly
projecting above. Polyps are green, while skeleton or coral produced is dark red due to
the deposition of iron salts and calcium carbonate.
iv. Corallium (Red coral): It is a dimorphic colony with upright branches having a rigid
axis supported by the delicate tissue of coenosarc from which the short polyps arise (Fig.
5). Polyps are of two types i.e. anthocodia and siphonozooids, which arise in
perpendicular to the central axis lined by gastrodermal tubes (could be polyp
coelenterons). Anthocodia are feeding zooids while siphonozooids bear gonads and help
in sexual reproduction. Spicules are compacted with the calcareous cement like
substance and produce hard axial skeleton. It is one of the precious red coral and found
in the Mediterranean Sea and the Sea of Japan. It is used for making jewelry.
v. Gorgonia (Sea fan): They form a large interwoven tree like structure with branches
arising from the common basal plate. Branches arise only in one plane. They bear
numerous slender polyps called anthocodia with 8 pinnate tentacles while siphonozooids
are absent. All branches are interconnected by small cross connections to form a mesh
like structure. Within the mesogloea there is a network of branching tubes called solenia
which are extensions of the coelenteron (Fig. 6). The skeleton is made up of horny
protein called gorgonin around which calcareous skeleton is secreted. They are expanded
during the day than during the night. As gorgonians grow bigger, many small animals
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and symbiotic algae grow and crawl on the surface and get shelter and food. The color of
the gorgonians depends on the colour of the calcareous spicules and symbiotic
zooxanthellae growing within the colony.
B. Subclass: Hexacorallia (Zoantharia):
a. Structure and formation of a Hexacorallian coral:
Hexacorallians include stony corals which are also called true corals; therefore they are
very important and contribute a lot in the formation of coral reefs and coral islands.
These are cosmopolitan in their distribution in marine water and secrete compact
calcareous exoskeleton. They have mesenteries and tentacles either 6 in number or in the
multiples of six. Tentacles are simple. They may be solitary or colonial. Coral polyps are
similar in structure to sea anemones but do not have siphonoglyph. The skeleton is
secreted externally by the epidermis of lower half of the column as well as by the basal
disc of the coral polyps (Fig. 7a). The cells secreting the hard skeleton are called
calicoblasts which is composed of calcium carbonate. So, each polyp is fixed in a cup
like structure called calice. Calice is that part of the coral which is in direct contact with
the basal ectoderm of the polyp. The shape of the calice may be cups like, conical or
saucer shaped. The flat base of the cup secreted by the polyp beneath it is called basal
plate and surrounds the polyp base, while the side walls of the cup are called the theca.
From the basal plate, sclerosepta arise vertically between the mesenteries (Fig. 7b).
Sclerosepta are made up of rod like skeletal elements called trebaculae. As sclerosepta
grows up, the lower part of the polyp is invaginated forming the blunt end ridges towards
the gastrodermal cavity. These ridges alternate with the grooves enclosing the
mesenteries. Sclerosepta help in fixing the polyp to the basal plate and also help protect
them from predation. Mesenteries are also the site of digestion, excretion and gonad
development. They have double layers of gastroderm enclosing a thin sheet of
mesogloea as discussed earlier in the previous chapter of mesenteries. . The mesenteries
are arranged in pairs. The space between each pair is called endocoel while the space
present between adjacent pair of mesenteries is called exocoel. The lower epidermis
secretes the skeleton which grows in the form of sclerosepta in between the endocoel in
the beginning but more can be added even in the exocoel as the polyp grows. The inner
ends of the sclerosepta are fused to form an irregular central skeletal mass called
columella. In some corals, there is a special type of growth producing narrow upright
pillars along the inner ends of the sclerosepta. These are called palli and are formed in a
circle around columella. The skeleton secreted by a single polyp is called a corallite and
is called a solitary coral (Fig. 8a and b). Solitary corals are constricted at regular
intervals. In colonial corals, a single polyp grows asexually by producing buds laterally,
or by vertical fission into two or more daughter polyps within the calice, and skeleton is
secreted by each polyp. Thus polyps producing colonial corals are joined with each other
laterally near the bases. These lateral connections have upper and lower layers of
ectoderm and gastroderm with the extensions of the gastrovascular canals.
In colonial corals, thousands of corallites are secreted by thousands of polyps growing
together by budding or splitting. As all the polyps are joined together by horizontal
partitions, the resulting combined hard skeleton produced by a colony together is called
corallum. Live polyps are always present over and above the dead calcareous corallum
secreted at the base by them. A few examples are given below:
i. Fungia (Mushroom coral): It is a solitary discoid coral. Planula larva develops into a
single stalked animal resembling a cup like coral. Adult animal has a single polyp with
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many tentacles. Once it grows, it breaks itself from the stalk and free coral grows and is
moved by the water currents. It can perform certain movements because of the presence
of cilia and by inflating coelenteron with the water. The theca (cup) is confined to the
lower surface. Sclerosepta as usual grow within the base of the polyp forming ridges and
grooves. As sclerosepta grow, they are fused in the center producing columella. Many
thin synapticulae grow horizontally in between each sclerosepta connecting adjacent
septa and close the interseptal spaces. It secretes coral which is convex on the upper side
and concave on the lower surface (Fig. 8a).
ii. Madrepora (Horn coral): It is a colonial coral with many perforations present within
the coral. Colony is branched like the horns with small polyps within cup like structures
separated from each other by perforated coenosteum (The lower surface of the coenosarc
secretes the skeleton between adjacent corallites, which is known as coenosteum).
Terminal polyps bear 6 tentacles while lateral polyps bear 12 tentacles. The polyps are
connected by the coenenchymal canals passing through the loosely secreted theca,
(Fig.9) and as the corallites have many pores, are called perforate corals.
iii. Astraea (Star coral) : The colony consists of numerous polygonal cups or theca.
Each cup encloses a polyp and is very close to each other and shares their walls (Fig.
10). Polyps are connected by coenenchyma only over the upper edge of the theca.
Coenenchyma is formed by calcification of the coenosarc and gives rise to individual
corallite which lie near by. Skeleton produced by them is very hard, stony and therefore
produce massive corals. There are no pores or perforations in the coral colony and thus
called an imperforate coral.
iv. Meandrina (Brain coral): Brain corals form extensive masses of hard structures.
The surface of coral is marked by long, wavy ridges and grooves almost parallel to each
other and resembles the grooves and ridges present on the surface of the human brain.
Here many polyps are fused together so that they have a fused calice at the base, sharing
a common fringe of tentacles, rows of mesenteries and sclerosepta. But the mouths of
these joined polyps remain separate.
v. Astrangia (white coral): It inhabits the waters of north Atlantic coasts. These corals
secrete calcareous cups into which the polyps live and delicate polyps can withdraw
inside the cups at the time of danger.
vi. Antipathes (Black coral): It is a black coral. It is tree like with upright, plant like
colonies (Fig. 11a and 11b). Polyps bear six tentacles, out of which two opposite ones
are longer than others. Stem and branches have thorny appearance and have flexible
axial skeleton within the polyps. Skeleton is composed of a black, non-collagenous
horny material called antipathin. Polyps may have 6, 10 or 12 mesenteries and two
siphonoglyphs, but without muscles. Because of the absence of muscles, these polyps
cannot contract immediately like other anthozoans.
3. Class: Hydrozoa:
Some animals of the class hydrozoa like Millepora also
contribute towards the formation of coral reefs.
i. Millepora (Fire coral or sting coral): It is a colonial animal and bears gastrozooids,
dactylozooids and ampullae (which produce medusae). The calcareous skeleton secreted
by zooids bears number of pores and is called coenosteum. Through these pores zooids
protrude out. A gastrozooid is always surrounded by many dactylozooids. The
dactylozooids are longer than gastrozooids and bear capitate tentacles bearing nematocyst
which are highly irritating to humans. Some species have special rounded depressions
called ampullae in which simple medusae are produced for the sexual reproduction. They
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always live in symbiotic relationship with zooxanthellae and thus corals produced are
brown in colour (Fig. 12a and 12b). They secrete enormous skeleton and thus make
significant contribution in the development of coral reefs.
4. Coral reefs:
Coral colonies grow in size either by budding of the new polyps or by vertical splitting of
the body of the polyp. All the polyps are joined together at their bases and the skeleton is
hardened by the growth of inter-trebaculae horizontal partitions. As a result live polyps
move towards the top regions of the colony and are found growing there. The skeleton
secreted by them also grows as each polyp help in the formation of corals. Scleractinian
corals play a major role in the formation of corals reefs. Scleractinian are the stony
corals which secrete enormous amount of hard external skeleton made up of calcium
carbonate.
As we have already discussed different types of coral forming animals, it is very clear
that some are solitary corals and majority of them are colonial corals. Different types of
colonial corals growing together along with other associated fauna form large heaps of
calcareous mounds of lime stone called coral reefs, the upper surface of which is near the
surface of the sea. Such large rigid structures called coral reefs are built up by many
generations of coral polyps. The coral reefs provide a natural habitat for diverse
organisms like, fishes, mollusks, chaetopods, arthropods and echinoderms which live
together in a perfect harmony in a most complicated ecosystem. Dead remains and shells
of these animals also contribute to the growing corals reefs and thus indirectly help in the
formation of massive coral reefs. It takes millions of years for a full fledged coral reef
system to establish itself. Majority of the coral reefs were formed during palaeozoic era.
The madreporarian corals (modern corals) replaced them in the coenozoic era and form
the majority of the existing reefs and islands present in the Indian and Pacific oceans.
A. Development of coral reefs: The development of coral reefs takes place in four
stages.
First of all a single coral polyp colony grows. Later on this colony is joined by other
colonies and this becomes a thicket which may consist of colonies of same species or
different species. Thicket, thus created attracts other animals, like crustaceans, mollusks,
echinoderms, fishes and barnacles etc., forming a different niche (Fig. 13a). The new
ecosystem thus developed will always have wear and tear of the fauna and associated
corals due to interaction among different animals. When so many different animals are
staying together, some of them may be causing harm or may be benefiting others. They
also create lot of debris and excretory wastes. Some animals may be utilizing the wastes,
so they further attract more animals for the food and shelter. This leads to further growth
of the substratum for coral growth and another stage is developed called coppice which
may be several meters across which leads to increase in the substratum for the growth of
the colonial animals. As a result there is more debris production thus increasing the
number of coral inhabitants, with the result a bank is developed which is known as a
coral reef.
As we have already discussed that there are many important factors which determine the
growth of corals, so obviously there will be a gradation in the type and frequency of the
corals at different depths of the sea starting from the sea shore as each species occupies a
specific habitat. Different regions of the coral reefs have different species, e.g. in some
regions, gorgonians are predominantly present while in other regions scleractinian corals
are present. For example the corals present at the inner reef flat, lagoon, outer reef flat,
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reef edge, and seaward slope will be different as all these different zones of the coral
reefs have different environmental conditions. This is called zonation of the corals, which
varies from reef to reef depending upon the environmental factors (Fig. 13b). .
B. Types and structure of Coral reefs
There are three types of coral reefs:
a. Fringing reefs
b. Barrier reefs
c. Atoll
a. Fringing reefs: Fringing reefs are developed in shallow waters on or near the shores
of the volcanic islands. These are the simplest kind of reefs. They extend from the
sea shore towards the sea as a platform ranging from few meters to half a kilometer
and then slopes down towards the bottom of the sea (Fig. 14). Fringing reefs consist
of several zones that are characterized by their depth, the structure of the reef, and its
plant and animal communities. If a section passing through the volcanic island and
the fringing reefs is observed, it shows different regions which can be
diagrammatically shown (Fig. 15). These regions differ from reef to reef and are not
fixed in their occurrence.
Reef edge or reef front: The edge of the reef where the coral growth is maximum and
it is slowing down towards the sea in a steep slope is called seaward slope. It is belt
like and broken here and there by water channels. It is covered by attached plants and
animals.
Seaward slope: Beyond the reef edge is a steep slope of the coral reef down to the sea
bottom. Live corals are present on this seaward slope from 40-100 meters deep
depending upon the penetration of light as many animals live in symbiotic
relationship with algae.
Reef flat: Reef flat can be inner reef flat and outer reef flat. Inner reef flat is the
region of the coral reef present between the shore and the reef edge. It contains coral
sand, detritus, dead remains of the coral colonies, and other animals. It may be 50 to
100 meters in breadth. It may take a form of a lake through which small boats can
pass through. Outer reef flat is another flat which may or may not present between
boulder zone and reef edge. The reef flat is generally submerged at high tide but it is
exposed at low tides.
Boulder zone: When water is flowing through the reef flats; stones, sand, pebbles,
coral sand, detritus, dead remains of the coral colonies, and other animals etc get
accumulated in an area called boulder zone and may be swept partly and are carried
across the reef flat by the surfs created by water currents.
Lagoon: It is the channel of water present between sea shore and reef edge of
volcanic island and another reef present within the sea water.
b. Barrier reefs. A volcanic island with the fringing reefs is surrounded with a big
channel of water called lagoon (Fig. 16). Its depth may vary from 20 to 100 meters
and even ships can pass through it. Lagoon is further surrounded by reefs called
barrier reefs. As their name suggests, they act as a barrier for ships between sea shore
and the main sea. Lagoon has fringing reefs towards the volcanic coast and barrier
reefs on the other side of it. Sometimes both fringing reefs and barrier reefs may join
each other at the bottom of the sea.
There is a Great Barrier Reefs of Australia. It is not a single structure but is made up
of many strings of separate reefs joined to each other at the bottom and thus forms a
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very big structure which extends along the north eastern coast of Australia for over
2000 kilometers. It is separated from the main land by a lagoon which is around 15 to
250 kilometers wide and 70 meters deep. During high tide, big ships can sail over it
without realizing that reefs are present nearby and may crash. The Great Barrier Reef
is the contribution of all different kinds of coral growth over the years.
c. Atoll: Atolls are coral reefs which are present within sea water hundreds or
thousands of kilometers away from the nearest sea shore. There is no volcanic island
present. It is more or less circular or horse shoe shaped reef enclosing a central
lagoon which may be 40 or 50 miles across and 20 to 90 meters deep. It may be a
complete or broken into many small reef islands separated from each other by water
channels (Fig. 17). At some places reef is low so sea water simply covers it and reef
is not visible. Sometimes a large atoll is formed by many small islets joined together
along a line of reef. Thousands of such atolls are found in the South Pacific. It must
be noted that reefs are not continuous rigid structures but they are broken up into
many reefs and islands by water channels. Suvadiva is the largest atoll present in
Maldives. Its circumference is about 195 kilometers and consists of around 102
separate little islets on its rim. Bikini Atoll has 2.87square miles land area with a
lagoon area of 280 square miles. It was inhabited by the people but people moved to
different places as it was selected by the United States for testing hydrogen and
atomic bombs. Horse shoe shaped atoll of West Texas is 70 to 90 miles across and
1,000 meters thick.
C. Theories explaining the formation of coral reefs: Many scientists tried to explain
the formation of coral reefs and gave their views which are discussed below in brief.
a. Darwin’s subsidence theory: Darwin believed that the reef began as fringing reefs
on a sloping shore. Then the island subsides very slowly, so slowly that the reef grows
upward at about the same rate, naturally the island becomes small, the channel between
the reef and land widens and thus the fringing reef transforms into a barrier reef. Further
subsidence of the land till it sinks completely out of site results in the formation of an
atoll (Fig. 18). This is substantiated by the fact that all the known coral reefs were in
regions where a sinking of the land was known to have taken place or where there were
evidences that it had probably occurred.
b. Stutchbury’s volcanic crator theory: According to Stutchbury, atolls of the pacific
were built upon the lost volcanoes. The crator of the volcano over the years got widened
by water currents and became the lagoon, while its raised edges of the land were grown
over by the coral reefs and formed atolls. This theory did not get much acceptance as
diverse shapes of the atolls with limited depths of lagoons and the great number of craters
in a single archipelago was present.
c. Samper Murray solution theory: This theory states that calcareous skeletons of
animals and other sediments form big mounds on the floor of the oceans. Over the time,
these deposits grow to certain heights and corals grow on them and reach the water
surface. Barrier reef is formed by the luxurious growth of coral at the outer edge while
atoll is formed by dissolution of the inner coral rock.
d. Submerged bank theory: This theory states that both barrier reefs and atolls grew
upon pre-existing flat surfaces. Extensive coral growth occurred on a flat surface which
got submerged in the water with the passage of time. Exposed regions formed the barrier
reefs, while the shape of the atolls is obtained by the action of prevailing water currents
and winds.
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Many boring experiments were performed in different coral reefs by many scientists to
find out the age and evolution of different coral reefs but no single theory could explain
the formation of coral reefs. Reef formation is not the result of one mechanism as many
different geological processes are occurring in the nature which might have led to the
formation of these heavy structures.
5. Significance of corals:
i. They show a high degree of physiological integration, division of labor, and
perfect coordination with other groups of animals staying together, mutually
benefiting each other in getting shelter, protection and food. Thus they constitute
an ecologically important aquatic ecosystem.
ii. They help in studying the evolution of the animals as fossils of the animals are
preserved in the coral reefs over the years.
iii. By studying the lines of growth on the fossils of some mollusk help in knowing
about the seasonal fluctuations as the thickness of the growth line varies from
season to season.
iv. A few stony corals because of the presence of minute pores are used in surgical
procedures as human capillaries can easily pierce through equal sized pores which
are helpful in interconnecting two bones with each other. These corals are being
used in the surgery for bone grafts and jaw surgery etc.
v. Horseshoe shaped Atoll is the largest limestone reservoir for the oil production in
North America.
vi. Many of the corals are precious stones which are used in making jewelry and have
aesthetic value.
vii. Coral reefs are very hard structures and are an important source of mortar,
cement, lime etc as they contain enough amount of CaCO3, therefore their rocks
can be used for making roads and houses etc.
viii.
Reefs are also a rich source for medical formulations, used to treat a wide
range of diseases like asthma, heart diseases, and viral, fungal and bacterial
infections. It has been reported in 2006 that Yellow coral (Isis hippuris) collected
off the coast of Okinawa island of Japan has yielded a compound that can slow
down and possibly prevent virus replication and also treat cancer.
ix. Lastly, corals act as affective buffers against erosion and storms occurring in the
sea thus help in preventing tsunami disaster.
6. Coral crisis: There are about 2500 living species of coral many of them are becoming
extinct due to change in the environment and global warming which is declining their
number. Coral growth is dependent upon many environmental factors as well as the
inhabitants of the coral reefs. Therefore, their growth is determined by the seasonal
changes like wave action of water, harm caused by boring organisms and predators
feeding on the coral polyps. These changes may be due to natural factors and man made
factors. Corals and coral reefs are extremely sensitive to slight changes in the reef
environment and which may have detrimental effects on the health of entire coral
colonies. There is a great loss to the species with the habitat loss and leads to the
extinction of many species.
Environmental factors change from season to season, therefore, coral formation and
deposition of the calcareous skeleton also varies. Besides this, there are certain predators
which feed on the coral forming animals, or form burrows inside the coral reefs and thus
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cause harm to the corals and produce solid calcareous debris on which algae grows, thus
sometimes preventing planulae larvae to form new colonies.
A. Natural factors affecting coral growth: Few factors which have a great impact on
the building of coral reefs are briefly given below.
a. Temperature: Corals are found in the tropical seas where the temperature never falls
below 200C at normal salinity. Therefore, they are absent from the region where
temperature goes down due to cold ocean currents coming from Antarctic.
A
temperature of 370C favors the growth of bacteria and viruses within the coral reefs
which damage the coral structures.
b. Depth of the ocean: Corals are differently located at various depths of the ocean. As
one goes down the ocean, temperature decreases and therefore they grow luxuriously at
the depth of about 30 meters but are scarcely found at the depth of 90 meters and more.
c. Availability of light and presence of algae:
Light availability also plays an
important role in the formation and growth of the corals. Their growth is comparatively
more in shallow waters as compared to the regions where light penetration level is veryvery low. This vertical distribution is restricted to the hermatypic corals (corals having
symbiotic algae i.e. dinoflagellates or zooxanthellae, growing in their endodermal cells)
as they have a significant role in coral formation. These algae grow in the presence of
sunlight, CO2, and water and release O2 and carbohydrates. The carbohydrates thus
produced are utilized as nutrients by the coral polyps while CO2 released by them is
consumed by the algae, thus mutually benefiting each other and getting rid of the
excretory wastes in a natural way. Therefore, hermatypic corals are restricted to shallow
warm waters where light penetration is sufficient for their growth. In contrast deep sea
corals i.e. ahermatypic corals have no zooxanthellae, and can grow up to the depth of
8000 meters They can grow even at 00C but are found growing in plenty within 5-100C.
They can live without light. Deep sea corals are solitary, which settles at the bottom, a
few are colonial and dendriform.
d. Sea storms: There are several natural disturbances which cause significant damage to
coral reefs e.g. hurricanes, tsunamis and storms, which bring large and powerful water
waves which break apart large corals and scatter them into fragments. After the storm,
these slow growing corals might easily be overgrown by quicker growing algae. In
addition, these storms generally bring heavy rain which increases runoff and
sedimentation e.g. latest disaster, the earth quake and tsunami of December 2004 caused
a great damage in the Andaman and Nicobar islands. Nearly 3,500people were reported
dead and thousands were left homeless. Extensive and beautiful coral reefs because of
which these islands were famous had a great damage. These reefs were hit because of the
submergence, the increased turbidity and the physical damage caused by the debris
thrown back and forth by the devastating waves. In the Andaman waters, huge coral
reefs were permanently exposed above the high tide line destroying them within weeks.
There is a not only reduction in the number of corals but other associated coral reef
inhabitants like fish, shrimps, flat worms and hermit crabs were also affected.
B. Man made factors: There are several factors which are dependent upon the humans
which contribute in destroying or damaging the coral reefs.
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a. Increase in human population and development: Increase in human population is
not only responsible to the depletion of natural resources but also causes harm to the
coral reefs. As population is growing, there is a need for the development of housing
societies and factories, with the result the amount of freshwater runoff increases carrying
large amounts of sewage, nutrients, pollutants like insecticides and fumigants from the
farm lands into the sea water. Sewage treatment facilities greatly increase the nutrients
and microorganisms while large power plants discharge extremely hot water along into
the coastal waters and increase the water temperature which may lead to white band
disease (WBD) or black band disease (BBD). Increase in turbidity decreases the amount
of light reaching the corals which may cause bleaching. Increase in nutrients may
enhance the growth of other boring animals and sponges which outgrow the corals for
space on the coral reefs. With the increasing human population, there is an increase in
the demand for the sea food, including reef fish, gastropods, bivalves etc. Due to over
fishing, overgrowth of the algae may take place which will not allow the planula larvae to
settle and so the coral reefs are not able to establish themselves. Sometimes, explosives
and poisons are used by the fishermen for the easy catch of the fish, which further
damage the corals in those areas.
b. Aesthetic value: Corals are very colorful and are also used in for making jewelry and
as show pieces for decorations. People either collect pieces of coral themselves or buy
them from the shops. Commercially they select most healthy colonies and sell them at
higher rate, thus making a great loss to the coral reefs.
c. Sea traffic: Coral reefs are also damaged by the leakage of fuel oils by large tankers
into the water. Sometimes large oil carrying vessels sink in the sea and oil spillage cause
mass destruction of the floating gametes and thus can effect coral reproduction and
development.
d. Predators: Boring chaetopods, gastropods, bacteria, viruses and starfish etc. cause
damage to the coral reefs. It has been reported that there is a large star fish i.e.
Acanthaster planci which take out its stomach on the coral and digest the living tissue
layer and cause drastic damage to the coral populations.
C. Coral Bleaching: Coral bleaching is the whitening of coral colonies due to the loss
of symbiotic zooxanthellae from the tissues of polyps. This loss exposes the white
calcium carbonate skeletons of the coral colony. Coral bleaching may be caused by the
expulsion of the algae from the polyps either due to shortage of nutrients or algae may
produce toxins under stress which affect the polyps. Coral bleaching may also be caused
due to coral diseases, excess shade, increased levels of ultraviolet radiation,
sedimentation, pollution, salinity changes, and increased temperatures. Coral bleaching
because of the warming has destroyed more than 90% of coral around Seychelles.
D. Coral Diseases: There are certain diseases which are reported to cause harm to the
coral reefs e.g. white band disease (WBD) and black band disease (BBD) which kill coral
tissue by growing in the form of a band around the coral and make them colorless and
lifeless. BBD is caused by cyanophyte Phormidium corallyticum, while WBD is
believed to be caused by a bacteria pathogen not known yet. These diseases weaken the
Page 100 of 115
corals and damage them. BBD has a higher rate of infection in warmer water so seasonal
temperatures affect the spread of BBD. The exact methods by which these diseases are
transmitted are unknown.
There are a great number of threats to coral reefs, and most of the threats are related to
humans directly or indirectly. Work must be done quickly to protect our threatened
aquatic ecosystem. There is a great need to enforce strict rules and regulations to ensure
that proper techniques of fishing are used without disturbing the coral life. Rules are of
no use unless people are properly educated throughout the world so that they are
willingly ready to protect natural resources and so the coral reefs.
Recently, Japan has begun planting baby coral on a Pacific atoll, to save sinking islets. It
is a multi-million-dollar project in which several plants of juvenile corals near the
uninhabited islets will be planted. So, it is a beginning not only to protect the islets from
disappearance but also to preserve the coral community.
Page 101 of 115
Mouth
Pinnate tentacle
Opening of
pinnate tentacles
Siphonoglyph
Solenia
Epidermis
Coelenteron
Spicules
Coenenchyma
Soft coral
Gastrodermal
tubes
1a
1b
Fig. 1a: Octocorallian polyp and coral structure.
1b: Different types of spicules found in mesogloea of octocorallian corals.
Page 102 of 115
Expanded polyps
(anthocodia)
Polyps withdrawn
within Enteron
Stalk of the
colony
Base for the
attachment
Fig. 2: Alcyonium coral colony
Polyps
Pinnate
tentacles
Coenosteum
Solenia
Blind cylindrical
cavities
Fig. 3: Heliopora coral showing two polyps and coral skeleton in a section.
Page 103 of 115
Expanded
polyps
Polyp
openings
Vertical tubes
Horizontal
platforms
Fig. 4: Tubipora, a part of the colony.
Fully
expanded
polyps
Pores in which
polyps are
withdrawn
Base of the
colony
Fig. 5: Corallium, a part of the colony.
Page 104 of 115
Anthocodia
Main Branches
Mesh formed by cross
connections between
smaller branches
bearing polyps called
anthocodia.
Central axis
Stalk
Basal disc
Fig. 6: Gorgonia colony
Page 105 of 115
Mouth
Epidermis
Tentacle
Gastrodermis
Stomodaeum
Gastrodermal
canal of another
polyp which is
being budded off
Mesenteries
Ridges
Sclerosepta
Corallite
Groove
Basal plate
7a
Theca
Primary
mesenteries
Retractor
muscles
Pharynx
Secondary
mesenteries
Endocoelic
Sclerosepta
7b
Fig. 7a: L.S. of a Hexacorallian coral showing the growth of sclerosepta alternating with mesenteries.
7b: Diagrammatic section of a polyp showing formation of sclerosepta within endocoels.
Page 106 of 115
Tentacles
Mouth
Coelenteron
Mesenteries
Sclerosepta
growing
inwards
Basal plate
Sclerosepta
Columella
8a
8b
Fig. 8a. Fungia (a solitary coral)
8b. Diagrammatic section of a solitary coral showing sclerosepta and mesenteries.
Corallites
Coenosarc
Polyps
Fig. 9: Madrepora (Acropora), a part of the colony.
Page 107 of 115
Polyps
Corallites
Solid and stony
coral
Fig. 10: Diagrammatic view of Astraea
Page 108 of 115
11a
Tentacles
Mouth
Retracted
polyps
11b
Fig. 11a: Antipathes, a single expanded polyp. 11b. A few polyps in retracted form.
12a
12b
Capitate
tentacles
Dactylozooids
Medusa
Dactylopore
Gastrozooid
Gastropore
Pores
Coenosarcal
canals
Basal disc
Degenerating
canals
Ampulla with
medusa
Tabulae
Fig. 12a: Millepora colony ( A hydrozoan coral). Fig. 12b: Millepora in section (magnified view).
Page 109 of 115
A polyp colony
Thicket
Coppice
Bank
Dead corals
and shells
Heavy coral
growth zone
13a
Reef edge
Reef slope
Sea ward slope
Coral reefs
Island
13b
Fig. 13a: Development of coral reefs
13b: Generalized diagram of formation of coral reefs.
Page 110 of 115
Island
Fringing reefs
Barrier reefs
Lagoon
Sea water
Fig. 14. Fringing reefs and Barrier reefs
Boulder zone
Fringing reefs
Inner flat
Outer flat
Reef edge
Seaward slope
Coral reef
Island
Coral reef
Fig. 15. Section showing Fringing reefs
Page 111 of 115
Low tide
High tide
Fringing reefs
Lagoon
Barrier reefs
Coral reef
Island
Coral reef
Fig. 16: Section through Barrier Reefs
Water
channels
Atoll coral reefs
Sea water
Coral reef
Coral reef
Fig. 17. Section through Atoll
Page 112 of 115
Coral reefs
growing up
Growing
corals
Subsiding
island
Island
Island
Lagoon
Coral
reefs
Cora reefs
Island
Fig. 18. Darwin’s theory to explain origin of coral reefs.
Page 113 of 115
VIII Bibliography
1. W. D. Russell-Hunter (1968), A biology of lower Invertebrates, The Macmillan
Company, Collier-Macmillan limited, London.
2. M. D. L.Srivastava (1964), A text book of Invertebrate Zoology, Central book depot,
Allahabad.
3. Robert D. Barnes (1986) Invertebrate Zoology, 5th Edition, ISBN 0-03022907-3
Saunders College Publishing, Address editorial correspondence to: 210 West
Washington Square, Philadelphia, PA 19105
4. Edward E. Ruppert, Richard S. Fox, Robert D. Barnes (2004), Invertebrate Zoology,
a functional evolutionary approach, 7th edition.. Thomson Learning Academic
Resource Center i-800-423-0563. ISBN0-03-02982-7
5. R.S.K. Barnes, P. Calow, P. J. W. Olive, D.W. Golding, J.I. Spicer (2001), The
Invertebrates: a Synthesis, 3rd edition; Blackwell Science Ltd Osney Mead, Oxford
OX2 0EL, 25 John Street, London WCIN2BS
6. N.P.O. Green, G.W. Stout, D.J. Taylor (1996), Biological Science, 2nd edition Edited
by R. Soper, Cambridge University Press,
7. Adam Sedgwick (1990) A student’s text book of zoology, Vol. I The Protozoa, by
professor J. S. Dunkerley, Manchester University, low price publication.
8. T. Jeffery Parker and William A. Haswell, (1962) A text book of zoology, 6th edition,
vol. I Macmillan & Co.Ltd, New York, St. Martin Press.
9. Richard A. Boolootian and Karl A. Stiles (1976), College Zoology, 9th edition.
Macmillan Publishing Co., Inc. New York, Collier Macmillan Publishers, London.
10. Jan A. Pechenik (2002), Biology of the Invertebrates, 4th edition. Tata McGraw-Hill
Publishing Company Limited, New Delhi.
11. E. N. K. Clarkson(1986), Invertebrate Palaeonotology and evolution 2nd edition,
Allen & Unwin(Publishers) Ltd, 40 Museum street, London WCIAILU, UK
12. Ashok Verma (2005), Invertebrates Protozoa to Echinodermata. Narosa publishing
house, 22, Darya Ganj, Delhi Medical association Road, New Delhi 110002.Brown,
B.E., Odgen, J.C. 1993. Coral Bleaching. Scientific American, 269:64-70.
13. Pechenik, J. A. 1991. Biology of the Invertebrates. Wm. C. Brown Publishers,
Dubuque, IA, pp. 91-92.
14. Rowan, R. and D. A. Powers. 1991. A Molecular Genetic Classification of
Zooxanthellae and the Evolution of Animal-Algal Symbioses. Science, Vol.
251:1348-1351.
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15. Sebens, K.P., Johnson, A.S. 1991. Effects of Water Movement on Prey Capture and
Distribution of Reef Corals. Hydrobiologia, 226:91-101.
16. Wilkinson, C. R. 1987. Inter ocean Differences in Size and Nutrition of Coral Reef
Sponge Populations. Science, Vol. 236:1654-1657.
IX Acknowledgements
I would like to express my deep thanks towards Dr. Narender, NISCAIR, for the confidence he
showed in me and gave me this opportunity to contribute in E-Books for the vast subject of
Zoology. I am also indebted to Ms. Shailley Anand, who showed a great patience in drawing the
figures included in this chapter. I earnestly thank my daughters Shalini and Deeksha who kept me
updated on my computer skills. I would also like to appreciate the technical support provided by
my brother Mr.Dinesh Sharma. I express my gratitude to Dr. Rajendra Prasad, Principal, Ramjas
College, University of Delhi, for all the encouragement during the course of writing these
chapters. My thanks are also due to my husband, Prof. Rup Lal for his continual support and
motivation in all the challenges that I have faced.
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