Download Lecture - 1 Ctenophora - Affinities, Type Study

CTENOPHORA
Course Name:
Zoology
B.Sc. 1st Year
Paper No. & Title:
Z-101B
Animal Diversity
Topic No. & Title:
A-5 Theory
Ctenophora - Affinities,
Type Study
Lecture No. & Title:
Lecture - 1
Ctenophora - Affinities,
Type Study
VIDEO INDEX : (PARTS OF THE VIDEO)
1) Introduction
2) Affinities and systematic position of Ctenophora
3) Examples of Ctenophora types
4) Summary
Academic Script
Introduction
The Ctenophora (Gr., Ktenos-Comb; phores-bearing) are marine, pelagic or
free, solitary and biradially symmetrical organisms, with transparent
gelatinous bodies. They lack nematocysts, possess ciliary plates in eight
rows and have a gelatinous ectomesoderm containing mesenchymal muscle
fibers. Their most distinctive feature is the "combs", groups of cilia they use
for swimming; they are the largest animals that swim by means of cilia –
adults of various species range from a few millimeters to 1.5 m in size.
There are peculiae adhesive cells in the tentacles. Skeleton is absent. All
the ctenophores have transparent gelatinous bodies and are commonly
called as “comb-jellies”.
Ctenophores are very common marine animals found in diverse habitat.
They are widely distributed being specially abundant in the warmer seas.
But some occur in temperate or arctic regions. They are of planktonic habit
floating in the surface waters, mostly near shores but few live to depth of
even 3000 metres. Ctenophores feed on small marine animals including the
eggs and larvae of molluscs, crustaceans and fish.
CLASSIFICATION
The phylum Ctenophore is divided into two unequal classes: Tentaculata
and Nuda. The class Tentaculata includes 4 orders:
(i) Cydippida
(ii) Lobata
(iii) Cestida and
(iv) Platyctenea.
The class Nuda has only one order: Beroida.
AFFINITIES AND SYSTEMATIC POSITION OF CTENOPHORA
In the field of taxonomic Zoology, Ctenophora, for a long time, occupies a
very important place. The importance is primarily due to its peculiar
anatomical organization. Martens (1671) first discovered Ctenophora.
Leuckart (1847-1848) included the sponges amongst the ctenophores. But
Hatschek (1889) gave a separate status to ctenophores as a group.
The ctenophores bear many characters of the Cnidarians, but differ
considerably from the other members of the phylum Cnidaria. This group
has also some similar features with different animals of diverse phyla,
which will be discussed here to judge its systematic position.
CNIDARIAN FEATURES
-
Possession of radial symmetry.
-
Lack of coelom.
-
Presence of gelatinous mesoglea.
-
Presence of ramified coelenterons.
-
Presence of diffused nerve network.
-
Presence of statocyst as sense organ.
-
Absence of organ systems.
-
Different parts of the body are arranged along and oral-aboral axis.
But the Ctenophora differs widely from the cnidarians by the possession of
meridional comb-plates, lack of nematocysts and possession of adhesive cells
(colloblasts). These have higher and complicated organization of the digestive
system, determinate type of development, direct development of muscle cells
from the mesenchyme and retention of cilia as locomotor organs in adult.
RELATIONSHIP WITH HYDROZOA WITH CTENARIA (ANTHOMEDUSA)
SIMILARITIES
-
Presence of two tentacles, situated at opposite per radii, each is provided with
a deep pouch at its base, revealing closely the tentacular sheath of
Hormiphora.
-
Presence of eight radial canals formed by the bifurcation of four inter-radial
pouches of the stomach.
-
The subumbrellar cavity of the ctenaria can be homologized with the
stomodaeum of Hormiphora.
DISSIMILARITIES
-
The gullet of Ctenophora is ectodermal in origin.
-
The tentacles of Ctenaria have no muscular base.
-
The development of gonad is different and develops from manubrium in
Ctenaria. But in Ctenophora gonads develop from meridional canals.
-
Absence of the characteristic of aboral sense organ in Ctenaria.
HYDROCTENA (NARCOMEDUSA).
Hydroctena, a trachyline medusa, shows close resemblance with Ctenophora.
-
Possession of two tentacles with sheath located between the margin and apex
of the bell.
But the presence of swimming plates in Ctenophora and the presence of velum
in Hydroctena remains as important differences to visualize a close relation
among them.
RELATIONSHIP WITH ACTINOZOA
The Ctenophores also possess certain anthozoan features such as:
-
Ciliated ectoderm of Anthozoa is probably a forerunner of the ciliated band of
Ctenophora.
-
Presence of well developed stomodaeum.
-
The gut in embryos of both is four lobed, thus, presenting biradial symmetry.
-
Mesoglea is cellular.
-
The gonads develop in connection with the endoderm and sexual elements
passed out through the mouth.
-
Both the aboral sense organ and rows of comb-plates of a ctenophore have no
parallel parts in an anthozoan. Lasso cells differ structurraly from the
nematocysts and tentacles are hollow in Anthozoa, while solid in Ctenophora.
RELATIONSHIP WITH SCYPHOZOA
The Ctenophores also possess certain Scyphozoa features such as:
-
The stomodaeum is similar.
-
Gametes are endodermal in origin.
-
Coelenteron is four-lobed. But due to the presence of four characteristic oral
arms, numerous marginal tentacles and the absence of meridional comb-plates
in Scyphozoa, the aforesaid homologies cannot be established.
RELATIONSHIP WITH SPONGES
Many authors tried to establish the sponges as to be closely related to
Ctenophora. Such as:
The large central cavity and the osculum of sponges correspond to the
coelenteron and mouth of Ctenophora respectively.
-
Absence of well formed mesoderm in both groups.
-
Simpler organization in both.
But closer examination reveals that these two groups are quite widely apart.
The differences are:
-
Developmentally the osculum of sponges does not correspond to the mouth of
Ctenophora.
-
Presence of inhalant pores and peculiar collar cells in sponges are absent in
Ctenophora.
-
Absence of colloblasts in sponges.
-
Absence of specialized nervous and sensory structures in sponges.
The above differences are sufficient to separate the two groups from each
other, though the above relationship is emphasized by Leuckart and some
other authors.
RELATIONSHIP WITH PLATYHELMINTHES
The idea that Ctenophora gave rise to certain bilateria (Polyclad) has been
supported by many Zoologists. Platyctenea has been considered to be a
connection link between Ctenophora and the Bilateria. Besides, Ctenophora in
general shows many structural similarities with the Platyhelminthes and
particularly with the Turbellarians. The similarities are:
-
General ciliation of the body.
-
The dorsal polar nerve of turbellaria can be compared with the statocyst of
Ctenophora.
-
Origin of the so-called mesoderm is more or less similar.
-
Primary locomotor organs in the larva (Mullaer’s larva) consist of eight ciliated
ridges of ectoderm which can be compared with the Ctenophora meridional
comb-plates.
-
Ctenophora exhibits both radial as well as bilateral symmetry types.
The view that the primitive Bilateria have evolved through Platyctenea, has not
been accepted. Because a thorough examination of the Platyctenea reveals
that it is a Ctenophore which has become extensively modified for sessile
habits. It can further there be suggested that Platyctenea is a tissue grade
diploblastic animal, whereas Polyclad is an organ grade triploblastic form.
Moreover, it is said that amongst the Platyhelminthes, the Acoela is the most
primitive group and not Polyclad. The Ctenophora, on the other hand, shows
no close similarity with the Acoela.
RELATIONSHIP WITH NEMERTINES
The larval form of nemertine (Pilidium larva) shows some similarities with
Ctenophores. In Pilidium larva locomotion is performed by lobed bands of cilia.
The aboral end of the body contains a cup like sense organ. But these
similarities are of no use as the anatomical organization in both shows many
diversities. Ctenophores lack mesoderm proper. Paired tentacles with basal
musculature are present in Ctenophora. The aboral sense organ in both differs
widely.
EXAMPLES OF CTENOPHORA TYPES:
1) Cestum: Cestum is commonly known as Venus’s girdle. It inhabitats seas
specially the Mediterranean. The body is greatly elongated horizontally in the
sagittal, and compressed in the lateral plane so that it appears ribbon-like
structure. It is usually green, blue or violet coloured. Out of eight comb-plates,
four are very small and the other four are continued all along the aboral edge
of the body. The base of two principle tentacles are large and are enclosed in
sheaths and numerous small tentacles spring from grooves and are continued
the whole length of oral edge. The small lateral tentacles line the oral ridge.
Mouth lies in the mid-oral edge. The only sense organ, statocyst, is found on
the aboral edge. Cestum swims mainly by graceful serpentine movements by
the muscular undulations of the body as well as by beating of comb plates.
2) Ctenoplana: Ctenoplana is small marine, solitary planktonic ctenophore
reported once of the West Coast of Sumatra and once among the islands to the
East of Papua. Its body is nearly circular in outline and flattened dorsoventrally. It measures about 6 mm in diameter. Its dorsal surface is mostly
olive green, brown or reddish in color. In the centre of aboral or dorsal surface
of its body is a sense organ with a statolith surrounded by a ring of small
ciliated tentacles. The mouth lies in the center of ventral or oral surface.
Tentacles are two, pinnate and retractile. The organs of locomotion are eight
small deeply sunk swimming plates.
3) Coeloplana: Coeloplana is a marine solitary Ctenophore found in the Red Sea
and on the Coast of Japan. It resembles in most of the features to Ctenoplana.
However, its body is oval and dorso ventrally flattened but elongated in the
tentacular plane. It measures about 60 mm in length. Mouth is ventral in
position. Tentacles paired and retractile. Statocyst is dorsally placed.
Swimming plates are not found. The fertilized eggs are attached to the oral
surface of the mother by the sticky secretion and each develop into a typical
cydippid larva with comb plates.
4) Beroe: Beroe is commonly known as sea mitres or mitre jelly fish. It is found
in great swarms and cosmopolitan in distribution. Its body is thumble-shaped
and measures about 10-20 cm in height. It is usually pinkish in color. The
rounded aboral end bears the sense organ surrounded by polar fields. The oral
end is truncated and it is occupied by a wide mouth. The greater part of the
body is occupied by the huge stomodaeum, infundibulum, per radial and
infundibular gastrovascular canals, etc. Tentacles are not found in any stage of
this life. A small stomach is placed near the sense organ. It is voracious
feeder, swallowing crustaceans and sometimes other Ctenophores even larger
than itself.
5) Pleurobrachia: Pleurobrachia is a member of the phylum Ctenophora and it is
commonly referred to as a sea gooseberry.Pleurobrachia has a pear shaped
body about 5-20 mm in diameter, and transparent. The mouth is situated at
the center of the oral pole and the opposite or aboral pole is occupied by a
complicated and special sense organ. This sense organ is modified statocyst
and acts as an organ of equilibrium. From the opposite sides of the broad end
hang two long tentacles provided with numerous little tag like process. The
surface bears eight equidistant meridional bands or swimming plates or coatae
starting from near the aboral pole and extending about two-thirds of the
distance towards the oral pole. The body is covered externally by a delicate
ectodermal epithelium. The epithelium of stomodaeum is ectodermal and that
of infundibulum and its canals are endodermal. Both ectodermal and
endodermal epithelia are ciliated. Their bodies are virtually transparent and the
many cilia refract the light, producing rainbow-like colors that can give the
false appearance of bioluminescence. The branched tentacles can be white,
yellow, pink or orange. They have no nematocysts (stinging cells). Instead, the
two long extensile branched tentacles are armed with colloblasts.
OBJECTIVES
To study Ctenophora- Affinity and types study.
SYNOPSIS
Ctenophora is a phylum of animals that live in marine waters worldwide.
Their most distinctive feature is the "combs", groups of cilia they use for
swimming, and are the largest animals that swim by means of cilia. Adults of
various species range from a few millimeters to 1.5 m in size.Ctenophores
are also characterized by the colloblasts found on the surface of their
tentacles. These specialized glue cells secrete a sticky substance that helps
ctenophores to catch prey. The phylum Ctenophora includes about 150
species which are categorized into two classes- Tentaculata and Nuda. The
tentaculata
comprises
of
4
orders:
Cydippida,
Lobata,
Cestida
and
Platyctenea. The class Nuda includes a single order: Beroida. The examples
of the phylum are: Comb jelly, Beroe, Ctenoplana, Pleurobrachia, etc.
FREQUENTLY ASKED QUESTIONS
Q1.
Both cnidarians and ctenophores have two cell layers, with an
acellular, jelly-like layer in between. True or False
A1.
True
Q2.
What is an example of Ctenophora?
A2.
Ctenophores, variously known as Comb jellies, Sea gooseberries.
Beroe, etc.
Q3.
What is Acoela ?
A3.
Acoela is a taxonomic order of very small invertebrate animals which
resemble flatworms. This order is treated either as a group within the
flatworms or as one of two classes within the phylum Acoelomorpha,
in which case Acoela contains the majority of that phylum's species.
QUIZ
Q1.
An older term that refers to both the Cnidaria and the Ctenophora is
___________, which means hollow gut.
Q2.
A.
Enteron
B.
Polymorphism
C.
Gastrovascular
D.
Coelenterata
Of the Eumetazoa listed below, the _______________ are generally
considered the most primitive.
A.
Chordata
B.
Rotifera
C.
Ctenophora
D.
Onychophora
Q3.
Q4.
Q5.
Q6.
Paragastric cavity of Sponges is lined by
A.
Choanocytes
B.
Amoebocytes
C.
Chromocytes
D.
Pinacocytes
Symmetry of Ctenophores is generally
A.
Bilateral
B.
Radial
C.
Biradial
D.
Absent
Comb plates of Ctenophores possess
A.
Cilia for locomotion
B.
Cilia for filter feeding
C.
Teeth for crushing
D.
Teeth for filtering
Nematocyst is a
Group of cell
Organ
Part of a cell d. Cell
ANSWER
Q1.
An older term that refers to both the Cnidaria and the
Ctenophora is ___________, which means hollow gut.
A1.
Coelenterata
Q2.
Of the Eumetazoa listed below, the _______________ are
generally considered the most primitive.
A2.
Ctenophora
Q3.
Paragastric cavity of Sponges is lined by
A3.
Choanocytes
Q4.
Symmetry of Ctenophores is generally
A4.
Radial
Q5.
Comb plates of Ctenophores possess
A5.
Cilia for locomotion
Q6.
Nematocyst is a
A6.
Cell
SUMMARY
-
Ctenophora commonly known as comb jellies is a phylum of animals that live
in marine waters worldwide. Their most distinct feature is the "combs", groups
of cilia they use for swimming.
-
The Ctenophores are characterized by having biradially symmetrical bodies,
eight meridional comb-plates on the body,mostly triploblastic, mesenchymal
muscles, an aboral sense organ and adhesive cells.
-
The comb rows of most planktonic ctenophores produce a rainbow effect,
which is not caused by bioluminescence but by the scattering of light as the
combs move.
-
The Phylum Ctenophora includes about 150 species which are categorized into
two unequal classes- Tentaculata and Nuda. The Tentaculata comprises of 4
orders: Cydippida, Lobata, Cestida and Platyctenea. The class Nuda includes a
single order: Beroida. The examples of the phylum are: Pleurobrachia,
Bolinopsis, Cestum, Ctenoplana, Beroe, etc.
-
The phylogenetic status of Ctenophora is controversial. They have possibly
diverged from the early trachyline stem from which holds the ancestry of
Cnidarians.
Tutorials
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Reference
1).
E.L. Jordan and P.S. Verma.
(2003). Invertebrate Zoology.
2).
Richard C. Brusca, Gary J. Brusca. (2003).
Invertebrates Zoology.
Links
 http://en.wikipedia.org/wiki/Ctenophora
 http://www.ucmp.berkeley.edu/cnidaria/ctenophora
 http://faculty.washington.edu/cemills/Ctenophores.html
 http://jellieszone.com/ctenophores.htm
GLOSSARY
MESOGLEA
Mesoglea
is
the
translucent,
non-living,
jelly-like
substance
found
between
the two epithelial cell layers in the bodies of coelenterates.
The mesoglea is mostly water.
STOMODAEUM
The Stomodaeum, is a depression between the brain and the pericardium in
an embryo, and is the precursor of the mouth and the anterior lobe of the
pituitary gland.
POLYCLAD
The
Polyclad
represents
a
highly
diverse
clade
of
free-living
marine
turbellarian flatworms. They are known from the littoral to the sublittoral
zone, and many species are common from coral reefs. Only a few species are
found in freshwater habitats.
BIRADIAL SYMMETRY
Biradial
symmetry
both radial and bilateral.
is
one
type
of body symmetry which
is