The Zoantharia, Hexacoralla, or "Helianthoid Polypes," are defined by the disposition of their soft parts in multiples of five or six, typically the latter, and by the possession of simple, usually numerous, tentacles. There may be no corallum, or rarely a "sclerobasic" one. Usually there is a "sclerodermic" corallum, in which the septa in each corallite, like the mesenteries, are arranged in multiples of five or six.

The above characters, though distinctive of the Zoantharia as an order, are not capable of universal application, since the disposition of the mesenteries in sixes cannot be always recognised, and the tentacles are in rare instances fringed with lateral processes ("pinnate").

The Zoantharia are divided into three sub-orders - the Zoantharia malacodermata, the Z. sclerobasica, and the Z. scleroder-mata; according as the corallum is entirely absent or very rudimentary, is "sclerobasic," or is "sclerodermic."

Sub-order I. Zoantharia Malacodermata

In this section of the Zoantharia there is either no corallum or a pseudo-corallum in the form of adventitious spicules scattered through the soft parts. The "actinosoma" is usually composed of but a single polype. (The term "actinosoma" is a very convenient one to express in the Actinozoa what "hydrosoma" expresses in the Hydrozoa - namely, the entire organism, whether simple or compound.)

There are three families in this section, of which the Actinidae will require a somewhat detailed examination, since they may be taken as typical of the entire class of the Actinozoa.