Phylum Ctenophora (General Characters)

 

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Phylum Ctenophora

 

The name of the phylum ” Ctenophora”  is  derived from two Greek words which are ktenos (meaning comb) and phoros (meaning bearing) as they posses 8 comb like plates for locomotion. These animals are commonly known as Sea walnuts or Comb jellies.

General characteristics of Ctenophora:

Ø  All ctenophores are exclusively marine, solitary, free- swimming or pelagic and do not show polymorphism.

Ø  They feed on plankton, swim by cilia. Bioluminescence (the property of living organism to emit light) is well-marked in cteno­phores.

Ø  External surface with 8 vertical rows of comb plates of fused cilia for locomotion (hence known as comb jellies).

Ø  Biradially symmetrical (the arrangement of comb plates gives the appearance of radial symmetry; the tentacles and branching of gastro vascular canals are of bilateral type) along an oral- aboral axis and body is transparent.

Ø  Tissue level of body organization is present.

Ø  Tentacles may or may not be present. When present, they are two in number and are solid and retractile.

Ø  Body is acoelomate and triploblastic with outer epidermis, inner gastro-dermis and middle mesogloea. The mesogloea is different from that of cnidaria as it contains amoebocytes, elastic fibres and muscle cells.

Ø  In ctenophore, instead of nematocyst, special type of adhesive cells called colloblasts (lasso cells), are present in the epidermis of tentacles which help in food capture (except Euchlora rubra which has nematocyst on tentacles and does not have colloblasts).

Ø  Digestive system 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.

Ø  Circulatory, respiratory and excretory systems are absent.

Ø  The nervous system is diffused as in cnidarians.

Ø  The aboral end (opposite end of mouth) bears a sense organ, called statocyst for equilibrium (balance).

Ø  They are monoecious (hermaphrodite or bisexual). Fertilization is generally external.

Ø  Asexual reproduction and alteration of generation is not present but regeneration and paedogenesis is common in them.

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Development is indirect with a larva called cydippid larva.

 

 

Ctenophora                       Cydipid larva