Hydroida

Hydroida, an order of Craspedote Hydrozoa (q.v.), including those in which the body is small, is usually fixed by an adherent base, and has a ring of tentacles around the mouth: they are rarely simple (e.g. Hydra) but are usually colonial. They are nearly all marine, but Hydra is fresh-water, and Cordylopltora (q.v.) has acclimatised itself to fresh or but slightly brackish water during the present century. The order is divided into three sub-orders: (1) the Eleutheroblastica, including the fresh-water Hydra and one or two doubtful genera; (2) the Gymnoblastica, in which the general colony (hydrosoma) is protected by a chitinous skeleton or "polypary," which does not, however, protect the polypites or individuals (Cordylopltora (q.v.) is ei very good type of this group); (3) Calyptoblastica or Thecaphora, in which the polypites are protected by expansions of the skeleton into hydrothecee or cups; these may be "sessile," being attached to the main tube as in Sertularia, or pedunculate, being borne on long stalks as in Campanularia; (4) Hydrocorallinee (q.v.), including those which form a massive calcareous skeleton.