User:Pooja srivastava/E-Museum/Cnidaria

= General characters of Cnidaria =


 * Coelenterata includes two animal phyla, the Ctenophora (comb jellies) and the Cnidaria (coral animals, true jellies, sea anemones, sea pens, and their allies).
 * The term coelenterate is no longer recognized as scientifically valid, as the Cnidaria and Ctenophora are placed at equal rank under the Metazoa with the other phyla of animals.
 * Phylum cnidaria contains over 10,000 species of animals found exclusively in aquatic and mostly marine environments.
 * The name Cnidaria comes from the Greek word "cnidos," which means stinging nettle. Cnidarians  got this name because their nematocysts eject barbed threads tipped with poison.
 * Cnidarians are multicellular animals.
 * They contain hollow body cavity  known as coelom
 * They are aquatic and mostly marine in nature.
 * They may be sedentary or free swimming.
 * Body wall consists of 3 layers of cells-Ectoderm, Endoderm and Mesoglea.
 * They show the phenomena of polymorphism. They exist in two forms-Polyp and '''Medusa.

Polyps-


 * 1) It is ssesile and asexual zooid.
 * 2) They are approximately cylindrical in shape and elongated at the axis of the body.
 * 3) In solitary polyps, the aboral end is attached to the substrate by means of a disc-like holdfast called the pedal disc, while in colonies of polyps it is connected to other polyps, either directly or indirectly.
 * 4) The oral end contains the mouth, and is surrounded by a circlet of tentacles.

Medusae-


 * 1) It is free swimming and sexual zooid.
 * 2) Medusae vary from bell-shaped to the shape of a thin disk, scarcely convex above and only slightly concave below.
 * 3) The upper or aboral surface is called the exumbrella and the lower surface is called the subumbrella.
 * 4) The mouth is located on the lower surface, which may be partially closed by a membrane extending inward from the margin (called the velum).


 * Cnidarians possess nematocysts which serves the function of paralysing the prey by injecting prey.
 * Reproduction is both asexual and sexual. The eggs develops into a cilliated larva known as planula.

/Some Common Examples/