ACTINIARIA The Actiniaria or Sea Anemo nes (fig. 8) constitute a large and varied group of Anthozoa which are closely related to the true corals (Madreporaria) but which contrast with the latter in a num ber of ways. No sea anemone forms any skeleton; none pro duces a colony, so that the polyp which results from the develop ment of an egg remains a single individual all its life except when it undergoes fission (see below). The polyps are of very variable dimensions, but the average order of size is relatively large, and certain giant species in which the individual may reach two feet in diameter are the largest Anthozoan polyps in existence.
The life of an anemone is rarely completely sedentary and the animal, although spending periods of varying length attached by its base to a foreign support, can readily creep away; or by inflat ing itself with water it becomes buoyant, and is moved elsewhere by the motion of the sea. No other series of Coelenterata, taken as a whole, offers a parallel to this creeping habit of the anemones.
The general build of many anemones is stronger and more muscular than that of most other Anthozoan polyps, and in par ticular the retractor muscles of the mesenteries and the circular muscle of the body-margin (sphincter) frequently attain a high degree of development. The variation in the external form of the polyp is very wide ; but even greater is that of the internal organs. The number of mesenteries, their arrangement, their relation to one another with respect to size, and the degree of specialization of their musculature, vary to such an extent that within limits imposed by certain fundamental principles, almost any combi nation may be represented among them. No anemones possess the characters of Alcyonarian polyps, however.
The distribution of anemones is worldwide. They occur at very varying levels from the littoral zone to depths such as 2,900 fathoms. None occurs in fresh water, but a few are able to colonize brackish areas. Little is known of their geological range except that they must be of very great antiquity.
Since sea anemones do not form colonies, one would expect to find that the habit of budding, so prevalent in some groups of Coelenterata, is not much in evi dence here. Such is actually the case. Asexual reproduction of other kinds, however, is of fre quent occurrence. In certain species rapid longitudinal division (fission) of the whole anemone into two more or less equal parts is a regular habit; the animal lit erally tears itself in two, the throat being cleft as well as the other parts; and by the regenera tion of a new piece of tissue at the torn edge a new individual is formed from each half. In other species fission of another kind takes place. Here a small fragment becomes separated from the edge of the parent's base (sometimes as the result of an actual tear, sometimes as the more gradual product of a process of constriction) and this, although it contains no tentacles, peristome or throat, develops into a per fect new anemone (fig. 9). In a few species the direction of fission is transverse.
The classification of anemones is complicated, and the quota tion of it in this connection would be uninstructive.