CCELENTERATA, se-la-te-reti, a phylum of animals distinguished from all others by the following aggregate of char acters: Its members consist of two layers of cells only, separated by a gelatinous layer, and exhibit a more or less pronounced radial sym metry. They possess peculiar nematocysts (q.v.) or stinging organs, which are developed from the cells of the outer and sometimes of the inner layer. Such cells are not certainly lcnown in any other group. Sense organs res2onsive to light and to sound or changes in emabnum may exist among the ccelenterates, but are of the very simplest character. The differentiation of nerve and muscle cells is indicated, but is as yet very imperfectly developed. There is but one body cavity, which serves the purposes both of digestion and of circulation. All ccelenterates are carnivorous. One opening serves the purposes both of the mouth and of the vent. The individuals may be free, in which case they are known as medusx or jelly fishes, or fixed. as with the coral individuals and the sea-anemones, when they are lmown as polyps. An alternation between the poly poid and medusoid generations is common, with the polypoid. generation asexual and the medusoid generation sexual. Either genera tion may be aborted or entirely absent. .It is not yet known which represents the original ccelenterate condition. The asexual genera tions multiply by fission, or more conunonly, by gemmation,.and these processes also may occur in the sexual forms. The gemmation often re sults in the formation of colonies. Both polyps and medusm may be either separate or colonial. In the colonial forms a high degree of di vision of labor among the zooids is often found, with a corresponding differentiation of form. A chitinous or calcareous exoskeleton is often found. The ccelenterata are classified as fol
lows: 1. Hydrozoa.— The sexual cells discharge directly to the outside. The digestive pouch opens directly to the exterior and is not sub divided by bands of the inner body layer. _ 2. Schyphozoa.— The sexual cells discharge into the digestive-circulatory pouch, which ap pears to open directly to the exterior, and is not subdivided by bands of the inner body layer. Filaments of the inner body layer may extend into the digestive pouch.
3. Anthozoa or Actinozoa.— The sexual cells discharge into the digestive-circulatory pouch, which opens into the outside through a tube lined with the outer body-layer, and is subdivided by bands of the inner body layer supporting this tube, the so-called mesenteries. Radial symmetry is partly replaced by bilateral symmetry. Medusw are unknown.
The Ctenophora (q.v.) are often included among the Ccelenterates, but they possess a middle germ layer and no nematocysts, and show affinities to the flatworms, so that they will be treated under a separate head.
The Hydrocoa include many jelly-fish, the common hydra and many other polyp-like forms, and certain so-called °corals.* The Schyphozoa include some polyps and many of the more familiar jelly-fishes. The Anthozoa include most corals, and the sea-pens and sea anemones.
Fossil Ccelenterata can be found even in the Lower Cambrian rocks. Not only the slceletons of the various calcareous forms, but the im pressions of the circulatory-digestive sac of the.f rail medusa, are found from a very early penod. The graptolites (q.v.) have been as sociated with the Ccelenterata by some writers, though others believe them to be Polyzoa. See