CTENOPHORA

· Ctenophora is a phylum of exclusively marine organisms, formerly included in coelenterates.

· Ctenophores (Gr., cten, comb; phero, to bear) are commonly known as sea walnuts, comb Jellies, or sea gooseberries.
· They are radially (biradially) symmetrical, diploblastic (or perhaps triploblastic) organisms with tissue level of organization.
· Ectoderm and endoderm are separated by cellular mesenchyme.
· Most of the ctenophores are transparent, gelatinous, and pelagic but a few genera are creeping.
· Bioluminescence (the property of a living organism to emit light) is well marked in ctenophores.
· They live from surface waters to a depth of about 3000 meters; about 100 species of ctenophores have been described so far.
· The body is gelatinous and extremely fragile; its form may be globular, pyriform, or bell‑shaped; some species resemble a ribbon.
· The axis of symmetry is determined by the mouth of the statocyst (the organ of equilibrium).
· The mouth leads into the flattened, elongated pharynx.
· With eight rows of ciliary plates (combs or ctenes) at some stage in their life history; comb rows controlled by a unique special sense organ.
· The comb‑plates are the main locomotory structures and are the most characteristic structure of the ctenophores.
· Each plate consists of a great number of very long related cilia.
· The ctenophores possess adhesive exocytotic structures called colloblasts (sometimes called lasso cells).
· The ctenophores feed on zooplankton.
· Gastrovascular cavity is the only "body cavity"; gut with stomodaeum and canals that branch completely throughout the body; gut ends in two small anal pores.
· Digestion is both extracellular and intracellular.
· Ctenophores are themselves important plankton organisms; they are quite common and have a worldwide distribution in all seas.
· Some genera stand great changes in the seawater salinity.
· Without respiratory, excretory, or circulatory systems.

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