Cœlenterata, the technical name for the second lowest alliance of many-celled animals. The class includes (1) locomotor types—jelly-fishes, swimming bells or medusoids, colonial medusoids such as the Portuguese Man-of-war, and extremely active organisms known as Ctenophores; (2) tubular forms or 'polyps' (mostly fixed)—the common Hydra, the compound Hydroids or zoophytes, the sea-anemones, compound sea-anemone-like types such as Dead-men's Fingers, and limy or coral modifications from all these. The two types, medusoid and polypoid, are inseparably linked together by real though not obvious resemblance in structure, and by the facts that many swimming bells are only detached and modified reproductive members of fixed zoophyte colonies, and that a fixed polypoid stage occurs as a chapter in the life-history of some common jelly-fishes.
General Characters.—(1) The Cœlenterates are nearer the primitive two-layered sack-like ancestral form, the Gastrula (see EMBRYOLOGY), than are higher animals; (2) they are radially symmetrical in almost all cases, that is, they are the same all round, and a line drawn vertically through the body of Hydra, Sea-anemone, or Jelly-fish corresponds to the long axis of the gastrula; (3) they do not possess any body-cavity distinct from the alimentary tube; (4) they have not a distinctly developed third or middle layer of cells, as in higher animals; (5) there is an almost constant presence of offensive stinging cells; (6) vegetative multiplication by budding is very common, and division of labour is frequent in the resulting colonial organisms; (7) the life-history often illustrates Alternation of Generations (q.v.); (8) 'coral'-forms occur abundantly by the formation of limy 'skeletons' in various passive types.
Classification.—(a) The Hydroids and the swimming bells often associated with them, the jelly-fish proper, colonial locomotor colonies like the Portuguese Man-of-war (q.v.), the limy Hydrocorallinæ (see MILLEPORES), &c., are grouped together as Hydrozoa. (b) The Sea-anemone and Dead-men's Finger types, with their associated 'corals,' form the division known as Actinozoa (q.v.). (c) The climax of activity is represented by the Ctenophora (q.v.), such as Berœ (q.v.). For details and fossil forms, see separate articles.
Habit of Life.—Five or six Cœlenterates, including the common Hydra, are fresh-water animals; the others are marine. A few are parasitic. Most of the active forms are found in the open sea, near the surface, but some live at great depths. The sedentary forms occur at all depths, and are often anchored on other animals, which they sometimes mask. The food of the great majority consists of small organisms, in the seizure of which the almost constant tentacles and stinging cells are very important.
Relationships.—For the connections between Hydra and the compound zoophytes, between the polypoid and medusoid types, between the medusoids and the Portuguese Man-of-war order, between medusoids and Ctenophores, between Sea-anemones and the sessile type exhibited in the life-history of some jelly-fishes, see separate articles (ANEMONE, BERÖE, CORAL, CTENOPHORA, HYDROID, JELLY-FISH, &c.).
Pedigree.—The ancestral Cœlenterate was probably gastrula-like. Modification has taken two main directions of increasing activity and increasing fixedness. The class as a whole probably arose apart both from the lower Sponges and from all the higher animals. See Huxley's Anatomy of Invertebrates, and other general works of Clans, Gegenbaur, Rolleston and Hatchett Jackson, Zittel, &c., and monographs noted under special articles.