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.
- It is ssesile and asexual zooid.
- They are approximately cylindrical in shape and elongated at the axis of the body.
- 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.
- The oral end contains the mouth, and is surrounded by a circlet of tentacles.
- It is free swimming and sexual zooid.
- Medusae vary from bell-shaped to the shape of a thin disk, scarcely convex above and only slightly concave below.
- The upper or aboral surface is called the exumbrella and the lower surface is called the subumbrella.
- 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.