In this family there is no corallum, and the polypes are single and free, with a rounded or tapering base. Ilyanthus itself (fig. 71, B) is in all essential respects identical with the ordinary Actiniae, but it is of a pointed or conical shape, the base being much attenuated, and it leads a free existence. Arachnactis (fig. 70, b) is also free, and according to Professor E. Forbes, it can not only swim like a jellyfish, but "it can convert its posterior extremity into a suctorial disc, and fix itself to bodies in the manner of an Actinia." It is by no means certain, however, that Arachnatis is a mature form, and there is some reason to suppose that it is merely the young stage of some at present unknown Actinozoon (perhaps of Edwardsia).
Edwardsia (fig. 71, A) has a thin imperforate base, and lives buried to the lips in mud or sand, the middle of the body being protected by an epidermic investment. This curious form exhibits, as shown by Prof. Allman, many singular peculiarities of internal structure, amongst which the fact that the soft parts are in multiples of four may be specially noted. The distinguished authority just mentioned regards Edwardsia as in some respects intermediate between the Zoantharia and Alcy-onaria, and as related to the extinct Rugosa. Peachia and Cerianthus also lived buried in the sand, both having the base perforated by an orifice, whilst the latter further protects itself by the secretion of a loose, membranous, non-adherent tube.
Fig. 71. - A, Edwardsia callimorpha; B, llyanthus Mitchelli, of the natural size.