Phylum Cnidaria

Phylum Cnidaria includes animals such as Hydra, Aurelia (jellyfish), Physalia (Portuguese man-of-war), Adamsia (sea anemone), Diploria (brain coral) and Gorgonia (sea fan). They are aquatic, mostly marine with a few freshwater forms, and may be sessile or free-swimming.

Cnidarians are radially symmetrical and diploblastic, following the blind sac body plan. The phylum shows two distinct body forms: a cylindrical, usually sessile polyp (as in Hydra), and an umbrella-shaped, usually free-swimming medusa (as in Aurelia, the jellyfish).

The single internal cavity serves both digestion and internal transport, so it is called the gastrovascular cavity or coelenteron. The tentacles surrounding the mouth bear specialised stinging cells called cnidoblasts (nematocysts), used for anchorage, capturing prey, and defence — these give the phylum its name. …