Habitat: All ctenophores are exclusively marine.
Habits: They feed on plankton, swim by cilia. Power of regeneration is well marked. Bioluminescence
Body form is variable
Symmetry:
Symmetry is biradial (radial + bilateral).
4. Habitat: All ctenophores are
exclusively marine.
Habits: They feed on plankton, swim
by cilia. Power of regeneration is well
marked. Bioluminescence
Body form is variable
Symmetry:
Symmetry is biradial (radial +
bilateral).
5.
6. The arrangement of comb plates
gives the appearance of radial
symmetry; the tentacles and
branching of gastro vascular
canals are of bilateral type.
5. Germ Layers:
They are diploblastic having
ectoderm and endoderm.
6. Level of Organisation:
Tissue level of organization is
present.
7. 7. Appendages:
Tentacles may or may not be present.
When present, they are two in
number.
8. Body Wall:
The body wall consists of outer
epidermis, inner gastro-dermis and
middle mesogloea (= collenchyma).
The mesogloea is different from that
of cnidaria as it contains amoebocytes,
elastic fibres and muscle cells.
8. From this reason ctenophores may be
considered as “triploblastic”.
9. Locomotion:
Comb like eight ciliary plates called
comb plates are present on the body.
The cilia of these plates help in
swimming. Ctenophores are hence
called comb Jellies.
10. Body Cavity:
They are acoelomates.
9. 11. Digestive Tract (Gastro vascular
tract):
It consists of mouth, pharynx or
stomodaeum, stomach or
infundibulum, anal canals and two
anal pores. The stomach is highly
branched to form a complex system of
gastro vascular canals.
Since there are mouth and anal pores,
the digestive tract is complete.
Digestion is both extracellular and
intracellular.
10. 12. Skeletal:
Circulatory, respiratory and excretory
systems are absent.
13. Nervous System:
The nervous system is diffused as in
cnidarians.
14. Sense Organ:
The aboral end (opposite end of
mouth) bears a sense organ, called
statocyst for equilibrium (balance).
11. 15. Reproduction:
They are monoecious (hermaphrodite
or bisexual). Fertilization is generally
external. Asexual reproduction is not
present. Paedogenesis is common.
16 Development:
Egg contains yolk, hence called lecithal.
Yolk is initially accumulated at the
centre (centrolecithal condition) but
later on when cleavage starts yolk shifts
to one side (telolecithal condition).
12. Cleavage is complete, holoblastic,
unequal, biradial and determinate
(a complete embryo is formed if all
the blastomeres remain together).
Gastrulation occurs by epiboly. The
development is indirect with a
ciliated sphencal cydippid larva.
13. Unique Features:
(i) Comb like ciliary plates for
swimming,
(ii) Special adhesive cells, the
colloblasts for capturing the
prey,
(iii) Mesogloea with amoebocytes
and smooth muscle cells,
(iv) Two anal pores.
14. Advancement over Cnidaria:
(i) Triploblastic origin of tissue,
(ii) Independent muscle cells,
(iii) Complete digestive tract,
(iv) Determinate Cleavage.
15. 7. Affinities of Ctenophora:
(A) Inter phylar Relationship:
Relationship with Sponges:
Many authors tried to establish
the sponges as to be closely
related to Ctenophora.
16. They put forward the following points
to support their contentions:
1. The large central cavity and the
osculum of sponges correspond to the
coelenteron and mouth of Ctenophora
respectively.
2. Absence of well formed mesoderm
in both,
3. Simpler organisation in both.
But closer examination reveals that
these two groups are quite widely
apart.
17. The points of differences are:
1. Developmentally the osculum of
sponges does in no way correspond to
the mouth of Ctenophora.
2. Presence of inhalant pores and
peculiar collar cells in sponges are
lacking in Ctenophora.
3. Absence of colloblasts in sponges.
4. Absence of specialised nervous and
sensory structures in sponges.
18. Relationship with Platyhelminthes:
Similarities between Polycladida
(Platyhelminthes) and Ctenophora:
The idea that Ctenophora gave rise to
certain bilateria (Polyclad) has been
supported by many zoologists.
Platyctenea has been considered to be
a connecting link between Ctenophora
and the bilateria.
19. 1. Flat compressed body having a
movement by creeping-like on the sole
of the foot.
2. General ciliation of the body.
3. The dorsal polar nerve of
Turbellaria can be compared with the
statocyst of Ctenophora.
4. Origin of the so-called mesoderm is
more or less similar.
20. 5. Primary locomotor organs in the
larva (Muller’s larva) consist of eight
ciliated ridges of ectoderm which can
be compared with the ctenophoran
meridional comb-plates.
6. Dermal musculature well developed.
7. Branched gastro-vascular cavity.
8. Ctenophora exhibits both radial as
well as bilateral symmetries.
9. Development of the two groups has a
close resemblance.
21. Dissimilarities:
1. Polyclads have a highly developed brain.
2. Numerous eyes in polyclads.
3. No definite ciliary plates in Turbellaria.
4. Tentacles generally absent in
Turbellaria.
5. Presence of flame cells in polyclads.
6. Ctenophores are radially symmetrical.
7. Complex reproductive system with a
muscular penis in polyclads.
8. Embryological details vary in these two
groups.
22. Relationship with Nemertines:
Similarities:
The larval form of Nemertine
(Pilidium larva) shows some
similarities with ctenophores.
1. Locomotion is performed by lobed
bands of cilia in Pilidium larva.
2. The aboral end on the body
contains a cup-like sense organ.
3. Radial structure without coelom.
4. Gonad ectodermal.
23. Dissimilarities:
1. Ctenophores lack mesoderm
proper.
2. Paired tentacles with basal
musculature are present in
Ctenophora, absent in Nemertines.
3. The aboral sense organ in both
differs widely because aboral
calcareous sense organ in
Ctenophora but absent in
Nemertines.
24. (B) Intraphylar Relationship:
Similarities with Cnidaria:
The Ctenophora bear many
characters of the cnidarians, but it
differs considerably from the other
members of the phylum Cnidaria.
25. Following features include them
within phylum Cnidaria:
Cnidarian features:
1. Presence of basic radial symmetry.
2. Lack of coelom.
3. Diploblastic body wall. The mesodermal
tissue is not distinct during embryogenesis.
4. Presence of gelatinous mesoglea.
5. Presence of ramified coelenteron.
26. 6. Presence of diffused sub-epidermal nerve
network.
7. Presence of statocyst as sense organ.
8. Absence of organ systems.
9. Arrangement of the parts of the body
along an oral-aboral axis.
10. Presence of tentacles.
11. Gastro-dermal origin of gonads,
12. Presence of lasso cells in ctenophores
similar to the nematoblasts of Cnidaria.
13. The body structure of ctenophores is
superficially similar to medusae of
cnidarians.
27. Dissimilarities:
The Ctenophora differs widely from the
Cnidaria by the possession of the following
features:
1. Possession of meridional combplates.
2. The symmetry of ctenophore is biradial
rather than radial.
3. Lack of nematocysts.
4. Presence of adhesive colloblasts or lasso
cells.
5. Presence of aboral sense organs.
6. Presence of only two tentacles in some
ctenophores.
7. Higher and complicated organization of
the digestive system.
28. 8. Direct development of muscle cells from the
mesenchyme.
9. Retention of multi-ciliated cilia as locomotor
organs in adult ctenophores but monociliated cells
in cnidarians.
10. Determinate (mosaic) type of development in
Ctenophora but indeterminate type of development
in cnidarians.
11. Gastrulation by epiboly or invagination in
Ctenophora but delamination, or invagination in
cnidarians.
12. ‘Cydippid’ is the common developmental stage in
Ctenophora but Planula in cnidarians.
29. Relationship with Hydrozoa:
With Ctenaria (Anthomedusa):
Similar features:
1. Presence of two tentacles, situated
at opposite per radii, each is
provided with a deep pouch at its
base, resembling closely the
tentacular sheath of Hormiphora.
2. Presence of eight radial canals
formed by the bifurcation of four
inter-radial pouches of the
stomach.
30. Dissimilar features:
1. The gullet of Ctenophora is ectodermal in
origin.
2. The tentacles of Ctenaria have no
muscular base.
3. The development of gonad is different.
The gonads develop from manubrium in
Ctenaria but in Ctenophora gonads develop
from meridional canals.
4. Absence of the characteristic aboral sense
organ in Ctenaria.
31. Relationship with Actinozoa:
Actinozoa exhibits some resemblances with
the ctenophores.
Similarities:
1. Transverse section of the embryos in both
of them are similar.
2. The stomodaeum can be homologised in
both groups.
3. The gonads in both are endodermal in
origin.
4. Mesoglea is cellular in both.
33. Relationship with Scyphozoa:
Similarities:
The scyphozoans have been looked
upon as to be related to Ctenophora.
Because in both of them the
following features are common:
1. The stomodaeum is similar.
2. Gametes are endodermal in origin.
3. Coelenteron is four-lobed.
34. Dissimilarities:
1. Presence of four oral arms.
2. Numerous marginal tentacles.
3. Absence of meridional comb-plates
in Scyphozoa.
