• CCELENTERATA: ACTINOZOA. Ii; 



Arachnactis is certainly free, and, according to Professor E. 

 Forbes, it can not only swim like a jelly-fish, but "it can con- 

 vert its posterior extremity into a suctorial disc, and fix itself to 

 bodies in the manner of an Actinia." It is by no means cer- 

 tain, however, that Arachnactis is a mature form, and there is 

 some reason to suppose that it is merely the young stage of 

 some at present unknown Actinozoon. 



Family III. ZoanthidjE. — In the Zoanthidce there is a spi- 

 cular corallum, and the polypes are attached by a fleshy or 

 coriaceous base or coenosarc. In Zoanthus the separate polypes 

 closely resemble small Actinics, but they are united together 

 at their bases by a thin fleshy coenosarc. 



Sub-order II. Zoantharia Sclerobasica. — The members 

 of this sub-order are always composite, and always possess a 

 corallum, but this is " sclerobasic," and there are no spicular 

 tissue-secretions. 



It appears advisable to explain here what is understood by 

 the terms "sclerobasic" and "sclerodermic," as applied to 

 corals. The "corallum" is the term which is applied to the 

 hard structures deposited by the tissues of any Actinozoon, 

 niany of which are so familiarly known as " corals." Usually 

 the corallum is composed of carbonate of lime; but it may be 

 corneous, or partly corneous and partly calcareous. Whatever 

 their composition may be, all coralla may be divided into two 

 sections, termed respectively "sclerobasic" and "sclerodermic," 

 which must be carefully distinguished from one another. The 

 " sclerobasic " corallum, of which the red coral of commerce 

 may be taken as the type, is in reality an exoskeleton, some- 

 what analogous to the shell of a Crustacean, being a true 

 tegumentary secretion. At the same time it is not a shell or 

 external envelope, but it forms an axis, upon which the entire 

 actinosoma is spread. The actinosoma, in fact, is inverted, 

 and the " sclerobasis " is secreted by the outer surface of the 

 ectoderm. The sclerobasic corallum is therefore truly " out- 

 side the bases of the polypes and their connecting coenosarc, 

 which, at the same time, receive support from the hard axis 

 which they serve to conceal." — (Greene.) Upon this view the 

 sclerobasis is termed " foot-secretion " by Mr Dana. In other 

 words, the sclerobasic coral is a hard skeleton which belongs 

 solely to the cmnosarc of the actinosoma, and which can there- 

 fore be produced by a compound organism only. 



The "sclerodermic" corallum, on the other hand, is secreted 

 within the bodies of the polypes, apparently by the inner layer 

 of the ectoderm — the " enderon " of Huxley — and it is there- 

 fore termed " tissue-secretion " by Mr Dana. In the sclero- 



