114 MANUAL OF ZOOLOGY. 



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 Actinozob'n, 

 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. 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 ccenosarc, 

 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. 



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

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



Fig. 30. Morphology of Corals, a Cup of Acervularia ananas, showing calicular 

 gemmation, enlarged ; b Diagram of Rugose Coral (Polycoelia profunda), showing 

 the quadripartite arrangement of the septa ; c Diagram of a recent coral, showing 

 the sextuple arrangement of the septa; d Vertical section of Campophyllumflexuo- 

 sum, showing tabulae. 



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

 fore termed " tissue-secretion " by Mr Dana. A sclerodermic 

 corallum, of course, like the animal which produces it, may be 



