Cnidarian

The Cnidaria (pronounced ny-DARE-ee-ah) are a phylum of simple ani­mals including the hydras, jellyfish, sea anemones, and corals. Any swim­mer who has suffered a jellyfish sting has painfully encountered the feature for which the phylum is named: the venomous, stinging organelles called nematocysts or cnidae (pronounced NID-ee). Nematocysts are used for de­fense and to sting and paralyze prey, ranging from plankton to fish.

Cnidarians have a simple body plan with two epithelial cell layers: the epidermis and gastrodermis, separated by a gelatinous mesoglea (“middle glue”). The mesoglea ranges from a thin, gluelike layer in the freshwater hydras to a thick, gelatinous layer in the jellyfish. The simple body wall en­closes a water-filled space, the gastrovascular cavity, responsible for the di­gestion of food and the distribution of digested nutrients.

Many cnidaria have a life cycle that alternates between a sessile polyp stage and a swimming medusa. The polyp may consist of a single stalklike body, attached to the substrate below and with a mouth surrounded by a ring of tentacles above; or it may be a branching colony, easily mistaken for a plant until one looks at it under the microscope. The medusa (jellyfish) is typically umbrella shaped, with a mouth-bearing stalk where the handle of the umbrella would be, and stinging, nematocyst-laden tentacles around the margin. Hydras, corals, and sea anemones, however, have only the hydroid stage, and some medusae have no polyp stage in the life cycle.

REFERNCES

Pechenik, Jan A. Biology of the Invertebrates, 4th ed. Boston: McGraw-Hill, 2000.

Rupert, Edward E., and Robert D. Barnes. Invertebrate Zoology, 6th ed. Forth Worth, TX: Saunders College Publishing, 1994.