I3O MANUAL OF ZOOLOGY. 



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

 Forbes, it can not only swim like a jelly-fish, 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 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. ZOANTHID^E. In the Zoanthida there is a 

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

 coriaceous base or coenosarc. The base is not muscular, and 

 they possess no power of locomotion. In Zoanthits the sepa- 

 rate polypes closely resemble small Actinitz, but they are united 

 together at their bases by a thin fleshy coenosarc. 



SUB-ORDER II. ZOANTHARIA SCLEROBASICA. The members 

 of this sub-ord^r 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^ 

 many 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, much as the bark of a tree encloses 

 the wood. The actinosoma, in fact, is inverted, and the 

 " sclerobasis " is secreted by the outer surface of the ectoderm. 

 The sclerobasic corallum is therefore truly " outside 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 ccenosarc of the actinosoma, and which can therefore 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 



