Phylum Coelenterata (Cnidaria) and Phylum Ctenophora

Phylum Coelenterata (Cnidaria). Members of this phylum are found almost entirely in aquatic, chiefly marine, habitats, living either fixed to a substratum (sessile, e.g. sea anemones and corals) or moving freely through the water (e.g. jellyfish). The body shows radial symmetry and a diploblastic, tissue-level organisation, with the two germ layers separated by an undifferentiated jelly-like mesoglea. Digestion takes place in a single central cavity, the coelenteron (also called the gastrovascular cavity, since it does the combined work of a gut and, in the absence of a true circulatory system, a distribution network), which opens to the exterior through one aperture that serves as both mouth and anus.

The name Cnidaria refers to the phylum's characteristic stinging cells, cnidoblasts (or cnidocytes), which are especially abundant on the tentacles surrounding the mouth. Each cnidoblast houses a coiled, thread-like stinging capsule called a nematocyst, which is discharged on contact or chemical stimulation and is used for anchorage, defence and the capture of prey. Coelenterates commonly occur in two structurally different body forms: a sessile, cylindrical polyp (such as Hydra, or a single coral individual) and a free-swimming, umbrella-shaped medusa (such as a jellyfish); in many species, such as Obelia, both forms occur in the same life cycle, one giving rise to the other in an alternation of generations that is called metagenesis. Familiar examples include Physalia (the Portuguese man-of-war, actually a colony), Adamsia (the sea anemone often found on the shell occupied by a hermit crab), Pennatula (sea pen), Gorgonia (sea fan), Meandrina (brain coral) and Aurelia (the moon jellyfish). …