Download Obelia

Obelia : the sea fur
Ms. Sneha Singh
Department of Zoology,
DAVCG, Yamunanagar.
CLASSIFICATION
Phylum
Class
Order
Suborder
Family
Genus
Species
-
Coelenterata
Hydrozoa
Hydroida
Leptomedusae
Campanularidae
Obelia
geniculata
Natural History
(1) HABITAT: small marine animal found in all seas from shallow
coastal waters to a depth of about 72 mts.
(2) HABITS: Obelia is a colonial form which is sedentary (fixed);
attached to various objects like rocks, piers (landing stage), seaweeds, shells and ships; Colony grows and looks like a developing
plant, hence, sometimes called Zoophytes; has more than one type
of individuals (polyp, blastostyle and medusa forms) called
Polymorphism; carnivorous in feeding, pray captured by special
cells called Stinging cells (Nematocysts) which also used for
defence; digestion intracellular and intercellular both; respiration
through body surface; luminescence (emits light) produced by
granules in the polyps; reproduces by sexual and asexual methods;
life history involves Alteration of generations including a freeswimming larva.
OBELIA COLONY
MORPHOLOGY OF THE COLONY
 Form, Size and Colour: Obelia colony gives a fur-like or mosslike appearance; grows to a height of 30 mm; creamy-white or
light-brown in colour.
 Body parts: Its colony consists of two types of branching
filaments – horizontal fixed branch called Hydrorhiza and
vertical branch arising form hydrorhiza called as Hydocauli
branching further in a scorpioid cymose manner. Each branch
gives rise to the next branch and itself ends into a zooids (Polyp,
blastostyle, medusae) thus giving a zig-zag pattern (see diagram).
The entire colony encloses a continuous cavity or
coelenteron so that food can be distributed throughout the body.
The wall of cavity has two layers, viz., the outer firm, chitnous,
nonliving Perisarc and inner soft and living celled Coenosarc.
 Obelia
colonyis trimorphic or shows polymorphism hence
develops three types of Zooids:(i) Polyps or hydranths or trophozooids,
(ii) Blastostyles or gonozooids, and
(iii) Medusae or gonophores.
(1)
Polyps (Hydranth, Trophozooid):- short, thick cylindrical
body with a prominent projection, the manubrium or
hypostome at the distal end bearing mouth and ring of about
30 long, tapering, filiform tantacles at surrounding its base thus
possessing radial symmetry. Perisarc of the polyp-bearing branch
expands to from a transparent, vase-like protective covering
called Hydrotheca. Hydroids having hydrotheca are called
thecate and those lacking it are called athecate.
 Mouth leads to a narrow passage called gastrovascular cavity.
 Histology:
Body wall of polyp is diploblastic, i.e., consists of
two layers of cells:- outer thin layer of cells, epidermis and
inner 2 times thick layer of cells called gatrodermis. Between
them a thin sheet of noncellular, jelly-like substance, mesogloea.
(1) Epidermis: The epidermis is protective, sensory, muscular,
secretory and reproductive in function. It comprises five types of
cells: (a) epitheliomuscular cells; (b) interstitial cells; (c)
nematoblasts or cnidoblasts or cnidocytes; (d) nerve cells or
neurons; (e) sensory or receptor cells.
(2) Gastrodermis: It is primarily digestive, absorptive and
muscular in function besides helping in circulation of food. It
also comprises of five types of cells: (a) epitheliomuscular or
nutritive-muscular cells; (b) gland cells; (c) nerve cells; (d)
sensory cells; (e) interstitial cells.
Functions: Polyp feed and defend the colony from predators.
PRINCIPAL TYPES OF CELLS IN THE BODY
WALL OF POLYP OF OBELIA
NEMATOBLASTS OR CNIDOBLASTS


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These are unique characteristic cells lying in the
epidermis of the tantacles and hypostome. A
nematoblast is a large, pear-shaped cell with its
nucleus lying at the base. It bears at its free end a
short, stiff bristles, the cnidocil. The cnidoblast
develops in it a remarkable chitinous structure
known as the nematocysts.
The nematocyst consists of a large ovoid,
double-walled capsule and a long, slender
thread-tube, operculum, number of short
retractile rods lie on the outer surface of the
capsule connected with long contractile fibrils,
the lasso arises form the base of capsule which
prevents the capsule from being thrown out at
the time of discharge. The capsule is filled with
poisonous protein fluid, the hypnotoxin.
They are protective in function and of one
time use.
(2) Blastosyle (Gonozooid):- is a long, narrow, club-shaped
zooid closed at the distal end and are developed on full-grown
hydrocauli, generally on their lower parts. They arise as buds in
the axils of some of the branches that bear polyps.
Blastosytle lacks mouth, manubrium and tantacles.
 Protective Covering: The blastostyle is enclosed in a clubshaped expansion of the perisarc called Gonotheca having an
aperture at its distal end called Gonopore. The gonozooid and
gonotheca together referred to as Gonangium.
 Histology: The body wall of blastostyle is similar to that of
polyp in having two layers of cells: outer epidermis and inner
gastrodermis and non-cellular mesogloea in between them. The
cells of these layers are also similar to polyp body wall except
that epidermis lacks stinging and sensory cells.
 Function: The blastostyle serves to produce third type of
zooids called Medusae, hence, called reproductive zooids.
(3)
Medusa (Gonophore):- They are superior to both the
polyps and blastosyles in structure as well as activity.
 Morphology: A medusa is a transparent, umbrella-like zooid,
just 1 mm. in diameter when first liberated and about 5-6 mm.
when mature. The upper convex, aboral surface is called as
exumbrella and the lower concave side is called as oral surface
or subumbrella. From the centre of the subumbrella hangs a
vertical tube called the manubrium (corresponds to hypostome
of the polyp) having mouth at its lower end. The edge of
umbrella projects inward as a narrow shelf called Velum and
various types of tantacles projects form its sides along the rim.

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Digestive Cavity: The mouth opens into a short cavity, the gullet,
present in the manubrium which further leads to a small cavity, the
stomach present at the centre of medusa. From the stomach arises
radial canals which extends toward the margin of umbrella equally.
At the distal end the radial canals open into a circular ring canal
running at the edge of umbrella. All these forms the Gastrovascular
cavity for the digestion and distribution of food.
Histology of Medusa: All the exumbrella and subumbrella parts
of medusa along with manubrium, velum and tentacles are covered
by epidermis while the entire gastrovascular cavity is lined with
gastrodermis and thick gel-like layer of mesogloea inbetween
forming bulk of medusa (hence called Jellyfish). The mesogloea
lacks cells but have elastic fibres. Gastrodermis is absent in velum
but forms solid core in tantacles. The tantacles and mouth contains
abundant stinging cells in their epidermis and interstitial cells at
their swollen base.
Musculature: The musculature of medusa is better developed and
more efficient than that of polyp for providing rapid and powerful
movements. It is almost entirely epidermal and consists of very
strong muscle processes of epitheliomuscular cells of subumbrellar
side. On this side there are one circular to few radial muscle bands
forming independent muscular tissue.
 Nervous System: Their nervous system is also superior to that of
polyp which includes sense organs and a double nerve ring in
addition to the double nerve net.
Sense organs includes Statocysts or marginal vesicles and
helps to maintain equilibrium while swimming. They are 8 in
number and lie one in the base of each adradial tentacle on the
subumbrellar side.
Nerve rings are formed by concentration or nerve cells, one
external to the circular canal and one just internal to it connected by
nerve fibres with tentacles, musculature and sense organs.

Life history of Obelia
 Fertilization:
External in Obelia and occurs in sea water. Sperms
and ova escapes into the sea from the gonads of the male and
female medusae by rupturing of epidermis. They come together
by chance contact.
 Development: It begins with cleavage which is holoblastic
(complete) and equal in Obelia conveting zygote into many celled
blastula. The various events takes place as shown in the diagram
ahead. The main feature in the life history of Obelia is formation
of Planula larva which is a ciliate larvae formed form fully filled
cells of blastomere called solid gastrula or stereogastrula.
 Polymorphism: The occurrence of more than one type of
individuals having different form, structure and function is called
polymorphism. The individuals in Obelia are called as Zooids
showing alternation in generation.
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