Phylum Cnidaria

Cnidaria (Greek knide, needle or sting) — formerly called Coelenterata — are aquatic animals that may be sessile or free-swimming, solitary or colonial, and are radially symmetrical except for sea anemones, which are bilaterally symmetrical. The phylum's name comes from cnidocytes (cnidoblasts), specialised stinging cells armed with a nematocyst on the tentacles, used for anchorage, defence and capturing prey.

Cnidarians are the first group of animals in the classification scheme to show tissue-level organisation, and they remain diploblastic. Their body surrounds a single central cavity, the coelenteron, which does double duty as both a digestive and a circulatory space, with a single opening (the mouth or hypostome) that handles both ingestion and egestion; digestion is both extracellular and intracellular. The nervous system is primitive, a diffused nerve net rather than a centralised brain. Some cnidarians, such as corals, secrete a calcium-carbonate skeleton. …