Coelenterates

Coelenterata is an historical, now obsolete term once used to group several aquatic animals believed to share a simple, radial body plan. The term often included the cnidarians—such as jellyfishes, corals, sea anemones, and hydras—and, in some classifications, the ctenophores (comb jellies). In modern taxonomy, Coelenterata is not regarded as a valid subdivision; the phyla Cnidaria and Ctenophora are treated as separate lineages within the basal metazoans.

Cnidarians are typically described as diploblastic, with two primary germ layers (ectoderm and endoderm) separated by a gelatinous mesoglea. They display radial symmetry and possess a simple gastrovascular cavity with a single opening that functions as both mouth and anus. A defining feature is the cnidocyte, a specialized cell that contains a nematocyst used for prey capture and defense. The nervous system is a diffuse nerve net rather than a centralized brain. Most cnidarians alternate between a sessile polyp form and a free-swimming medusa form, though anthozoans are largely restricted to the polyp stage.

Reproduction occurs sexually or asexually, depending on species, with life cycles that may include alternation of generations. Ecologically, many cnidarians play crucial roles in marine ecosystems; reef-building corals secrete calcium carbonate skeletons that form complex habitats, while jellyfishes and hydras contribute to food webs and nutrient cycles. Although historically grouped with ctenophores under Coelenterata, comb jellies are now recognized as a separate phylum, and contemporary classifications place cnidarians and ctenophores in distinct, early-branching lineages.