Actiniaria

(invertebrate zoology) The sea anemones, an order of cnidarians in the subclass Zoantharia.

An order of the Zoantharia known as the sea anemones, which are the most widely distributed of the anthozoans. Usually they are solitary animals which live under the low-tide mark attached to some solid object by a basal expansion or pedal disk. They feed on various prey such as copepods, mollusks, annelids, crustaceans, and fish. The burrowing species lack a pedal disk and bury their elongated bodies in the soft sediment of the oceans.

The freely retractile, skeletonless polyp has a cylindrical body, with a thick, tough, rough column wall often bearing rugae, verrucae, tubercles, or suckers (see illustration). The body is often encrusted with sand grains, pebbles, and other detritus. Some species have smooth, thin walls. Nematocysts discharge a toxic substance; however, the human skin is seldom affected by this. The colors of anemones vary with species and many variations occur even among the same species. The tentacles increase in number regularly and are arranged in several cycles. There are 6 primary, 6 secondary, 12 tertiary, 24 quaternary, and so forth in the hexamerous type. The musculature is the most highly developed in the coelenterates.

Actiniaria: Anthopleura sp. (a) Young, (b) Adult.

Most actinians are dioecious. Developing larvae pass through the Edwardsia stage then the Halcampoides stage. Longitudinal fission frequently occurs as well as budding. Sometimes new individuals result from laceration. See also Coelenterata; Zoantharia.