Whilst these terms must disappear from the systematic part of Zoology, they may still be employed, however, in the description of a local fauna or coral reef to signify the soft solitary zooids on {366}the one hand, and the organisms, animals or plants, which form large, massive skeletons of carbonate of lime, on the other. The form of the solitary zooids and of the colony of zooids in the Zoantharia, then, may be very divergent. In the Actiniaria we find single soft gelatinous zooids of considerable size adherent to rocks or half-buried in the sand. Among the Madreporaria we find great branching colonies of thousands of zooids supported by the copious skeleton of carbonate of lime that they have secreted. Among the Antipathidea, again, we find a dendritic skeleton of a dark horny substance, formed by a colony of small zooids that cover it like a thin bark. The majority of the Zoantharia are, like other zoophytes, permanently fixed to the floor of the ocean. Where the embryo settles, there must the adult or colony of adults remain until death. Some of the common Sea-anemones can, however, glide slowly over the surface on which they rest, and thus change their position according to the conditions of their surroundings. Others (the Minyadidae) float upside down in the sea, and are carried hither and thither by the currents. Others, again (_Cerianthus_, _Edwardsia_, _Peachia_), burrow in the sand or mud at the sea-bottom. The structure of the zooid varies considerably, but in the following characters differs from the zooid of the Alcyonaria. The TENTACLES are usually simple finger-like processes, and when they bear secondary pinnae these can readily be distinguished from the rows of secondary pinnules of the Alcyonarian tentacle. The number of tentacles is very rarely eight (young _Halcampa_), and in these cases they are not pinnate. The number of tentacles may be six (many Antipathidea and some zooids of _Madrepora_), twelve (_Madrepora_), some multiple of six, or an indefinite number. In the Thalassianthidae and some other families of Actiniaria the tentacles are plumose, but do not exhibit the regular pinnate form of the tentacles of Alcyonaria. [Illustration: FIG. 162.—Large (A) and small (B) plumose tentacles of _Actinodendron plumosum_. Large (C) and small (D) plumose tentacles of _A. glomeratum_. (After Haddon.)]