Phylum Cnidaria (Coelenterata)

Phylum Cnidaria (Coelenterata)

Cnidarians are radially symmetrical, aquatic (mostly marine) animals with a simple, sac-like body plan built around a single central gastrovascular cavity with one opening that serves as both mouth and anus, surrounded by tentacles armed with stinging cells (cnidocytes/nematocysts) used for capturing prey and defence.

Two body forms occur in the phylum: the polyp, a sessile, tube-shaped form typically attached at its base with the mouth and tentacles facing upward (as in Hydra and sea anemones), and the medusa, a free-swimming, umbrella- or bell-shaped form with the mouth and tentacles facing downward (as in jellyfish). Some cnidarians exist in only one of these two forms throughout their life, while others alternate between them.

Metagenesis refers to this alternation of generations between the two body forms within a single life cycle — a sessile, asexually reproducing polyp generation gives rise, at some point in the life cycle, to a free-swimming, sexually reproducing medusa generation, and the medusa in turn produces the next polyp generation sexually. This alternation of an asexual and a sexual phase, each with a distinct body form, is a hallmark feature of many cnidarian life cycles (well illustrated in Obelia).

Polymorphism is the occurrence of two or more structurally and functionally distinct individual types ("zooids") within a single colony of genetically identical individuals — for instance, a colonial hydrozoan may possess separate feeding polyps, defensive polyps, and reproductive polyps (or medusae) all budded from the same colony, each specialised for a different task, rather than every individual in the colony being identical and performing all functions itself.

The phylum is divided into three principal classes:

• Class Anthozoa (Actinozoa) — exclusively marine and exclusively polypoid (no medusa stage at all); includes sea anemones and corals, many of which are solitary or colonial and, in the case of reef-building corals, secrete a hard calcareous skeleton. …