Ctenophora

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).

1. Phylum : Ctenophora General Characters Dr. Rama Rao Kalinga M.Sc (Zoo); M. Sc ( Psy ) M.Ed ; Ph. D; PDF (DST); PDF (DBT ); D. Litt

2. 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).

3. 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 .

4. 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.

5. 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 .

6. 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.

7. 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 ).

8. 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).

9. 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.

10. 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.

11. Advancement over Cnidaria : ( i ) Triploblastic origin of tissue, (ii) Independent muscle cells, (iii) Complete digestive tract, (iv) Determinate Cleavage.

12. 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 .

13. 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.

14. 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.

15. 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 .

16. 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.

17. 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.

18. 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.

19. 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 .

20. 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 .

21. (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 .

22. 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 .

23. 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.

24. 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.

25. 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.

26. Relationship with Hydrozoa: With Ctenaria ( Anthomedusa ): Similar features: 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 . Presence of eight radial canals formed by the bifurcation of four inter-radial pouches of the stomach.

27. 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 .

28. 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.

29. Dissimilarities: 1. Presence of aboral sense organ in ctenophora . 2. Presence of hollow tentacles in Actinozoa .

30. 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.

31. Dissimilarities: 1. Presence of four oral arms. 2. Numerous marginal tentacles. 3. Absence of meridional comb-plates in Scyphozoa .