PCB Notes: Phylum-Ctenophora

Sunday, October 15, 2017

Phylum-Ctenophora

Ctenophores are commonly called comb-bearers.
Ctenophora derived from two greek words Ktenes- comb and phores- bearing.
Also called comb-jellies and sea walnuts.

General characteristic features of phylum- Ctenophora

Habitat- Ctenophores are exclusively marine and free swimming animals.
Body Symmetry- radially symmetrical.
Level of organization - Like Cniderians, they also exhibit tissue level of organization.
Germ layers- diploblastic . Outer ectoderm and inner endoderm are present with an undifferentiated layer mesoglea in between them. Mesoglea gives jelly like appearance to these animals.
Body form- They have a transparent body. Body shape varies from flat to oval. External surface of the body bears eight median comb plates. These comb plates bears cilia which are fused to make these plates ciliated. The ciliated comb plates help in locomotion.
Tentacles may be present or absent. When present the no. of tentacles is 2. They are solid and possess adhesive cells called colloblasts (lasso cells). Digestion is both extracellular and intracellular . Bioluminescence is well marked in ctenophores.
Digestion- both intracellular and extracellular. The food is captured by tentacles and directed to the gastro-vascular cavity of body. First the digestion takes place in the cavity then inside the food vacuoles.
Gastrovascular cavity is branched and opens to the exterior of stomodaeum.
Presence of a special sense organ 'Statocyst' at the opposite end of the mouth is the characterstic of the members of this phylum.
They are diploblastic animals but the mesoglea differs from cnideria; it contains amoebocytes and smooth muscle cells and is comparable to a loose layer of cells. From this viewpoint, Ctenophores may be considered as triploblastic.
Reproduction- Ctenophores are hermaphrodite. Sperms and ova are produced by the same individuals. Asexual reproduction is absent. Reproduction only takes place by sexual means.
Fertilisation- The fertilisation is external as fusion of gametes takes place outside the body of animal, in water.
Development - After fertilisation , zygote develops into a free swimming cydipped larva which later develops into adult . Hence the development is indirect.
Bioluminescence - Bios means living and luminesence means light . It is the property of production and emission of light by living organisms and hence shine in the dark background.
Examples- Pleurobrachia( sea gooseberry), Ctenoplana, Hormiphora (sea walnut), Cestum(venous girdle), Biröe.