CCELENTERATA : ZOANTHARIA. 159 



complicated skeleton developed in its interior. The animal 

 possesses a base, a column, and a disc the latter surrounded 

 by tentacles, and perforated centrally by the mouth. The 



B 



Fig' 73- Sclerodermic and sclerobasic Corals. A. Branch of Dendrophyllia nigres- 

 cens, a sclerodermic coral, showing the cups or thecse (a a) secreted by the separate 

 polypes, and united by the coenenchyma (c c). B, Portion of a sclerobasic coral 

 (Gorgonia) represented diagrammatically : * The solid and branched sclerobase ; b 

 A portion of the soft coenosarc with its embedded polypes, investing the sclerobasic 

 axis. 



mouth opens into a stomach-sac, connected with the body- 

 walls by mesenteries ; and the tentaculate disc and dependent 

 gastric sac remain permanently soft and capable of contraction 

 and expansion. Below the stomach, the soft tissues of the 

 polype are strengthened and supported by a more or less per- 

 fect calcareous skeleton or corallum (fig. 74). This is com- 

 posed of calcareous matter ("sclerenchyma") deposited by 

 and in the tissues themselves, between the endoderm and 

 ectoderm, and the corallum is thus ivithin the polype, and is 

 technically said to be "sclerodermic." The "sclerodermic" 

 corallum is therefore a true " tissue-secretion," anol thus differs 

 conspicuously from the " sclerobasis " of the Antipathidce and 

 Gorgonidce, which is secreted by the coenosarc, and is not formed 

 by a calcification of the soft parts of the polypes themselves. 

 The general distinction, arising from their mode of formation, 

 between " sclerobasic " and " sclerodermic " corals, is not, per- 

 haps, of essential importance, and the boundary-line between 

 the two is not very clearly marked ; but it is of considerable 

 practical value. It is, moreover, a distinction which is readily 

 recognised, as a rule, by a simple inspection of the corallum 

 itself. A sclerobasic corallum, namely, being secreted solely 

 by the coenosarc, never exhibits any parts which correspond 



