Cnidaria:
A group of diploblastic animals with radial symmetry without organ systems and annus and having single tubular gastrovascular cavities and with tentacles and nematocysts is recognized as Cnidaria.
Characteristics of Cnidaria:
1. Cnidarians are multicellular animals and the first metazoa with tissue grade of construction.
2. Most of the Cnidarians are marine, though some are freshwater forms.
3. Some of them remain attached to the substratum, some are free swimming, some live singly and some may form colonies.
4. Body radially symmetrical, some are biradial.
5. A single coelenteron or gastrovascular cavity is present which opens through the mouth, the anus is absent.
6. Small and narrow tentacles surround the mouth in a circle. Tentacles are used to procure food and for defense.
7. Body is covered by the epidermis, the gastrovascular cavity is lined by the endodermis, and in between the two, there is a layer of non-cellular matrix is known as mesoglea.
8. Body diploblastic and so coelom isn’t present.
9. The colonies show polymorphism with polyp and medusa as basic zooids. Polyp represents the asexual form and medusa represents the sexual zooid.
10. Nervous system consists of a nerve net with scattered nerve cells.
11. A special type of stinging cell known as nematocyst is present. These cells help in defense and food catching.
The nematocyst is provided with cnidocil from which the phylum name has been coined.
12. Reproduction is both sexual and asexual.
13. Ciliated planula larva is found.
14. In the life cycle alteration of generations and metagenesis are found.