252 CCELENTERATA. 



of the polype comes to lie external to the theca, while a portion lies 

 within the latter. It follows from this that the general body-cavity or 

 " ccelenteric space " is divided by the theca into two parts, of which one 

 is intrathecal, while the other is extrathecal, the latter being divided into 

 chambers by mesenteries, just as the former is. [According to von Koch, 

 the originally continuous mesenteries are cut into two by the fusion of the 

 peripheral ends of the septa to form the theca, each mesentery thus 

 becoming divided into an extrathecal and intrathecal portion.] Above 

 the lip of the calice, the extrathecal and intrathecal portions of the cce- 

 lenteric space communicate with one another. It is the extrathecal por- 

 tion of the polype which forms the " Randplatte" (fig. 131, r r) ; and the 

 so-called " epitheca," when present, seems to be formed " from the free 

 edge of the soft tissues on the exterior of the corallum, as they retreat 

 farther and farther from the original surface of attachment " (Bourne). 

 In composite corals the " ccenosarc" is the result of the coalescence and 

 union of the " Randplatten " of adjacent polypes ; and the general cal- 

 careous tissue ("ccenenchyma") which unites the various corallites is 

 secreted by the ccenosarc. Where a ccenenchyma is present, the ex- 

 trathecal portions of the mesenteries are awanting. 



The above gives the general structure of a typical simple sclero- 

 dermic corallum, as secreted by a single polype. A compound 

 sclerodermic corallum is the aggregate skeleton produced by a col- 

 ony of such polypes, and varies in form and size according to the 

 characters of the colony by which it is produced. In general, such 



Fig. 132. — Astrcea pallida, a compound sclerodermic Coral, in its living condition. 

 (After Dana.) 



a colony consists (fig. 132) of a number of polypes, which may 

 spring directly from one another, or may be united by a common 

 flesh or ccenosarc ; and corresponding elements are found in the 

 corallum. In the former instance, the compound corallum con- 

 sists of an assemblage of separate " corallites," as the skeletons of 

 the individual polypes are called, these being united with one 

 another directly and in various ways. In the latter instance, the 



