ACTINOZOA. 



253 



corallum consists of a number of " corallites " together with a com- 

 mon calcareous basis or tissue, which unites the various corallites 

 into a whole, is secreted by the ccenosarc, and is known as the 

 " ccenenchyma." 



The corallites of a composite sclerodermic coral are essentially 

 similar in structure to a simple corallum, such as has been previously 

 described. The "ccenenchyma" is the name given to all those 

 calcareous structures which may unite the different corallites with 

 one another, and which is, therefore, of " exothecal " or " peri- 

 thecal " origin ; and its nature varies greatly in different cases. Most 

 usually, however, the ccenenchyma consists either of simply porous 

 or of compact sclerenchyma ; though it may have the form of a 

 reticulated or vesicular tissue. In many of the composite coralla, 

 the corallites composing the colony are also invested inferiorly and 

 laterally by a general calcareous membrane or " epitheca," which is 

 common to the entire growth. 



A compound corallum is, of course, primitively simple, and it becomes 

 composite either by budding or by cleavage of the original polype, the 

 following being the principal methods in which this increase is effected. 



1. Lateral or parietal gemmatio7i. — In this mode of increase the original 

 polype throws out buds from some point on its sides between the base 



Fig. 133.— A branch of the recent 

 Dendrophyllia 7iigrescens showing la- 

 teral gemmation, a, A corallite ; c, 

 Ccenenchyma. 



Fig. 134. — Calicular gemmation as 

 seen in Lonsdaleia florlformis. Car- 

 boniferous. 



and the circle of tentacles, and these buds, on becoming perfect coral- 

 lites, may repeat the process. Commonly the new corallites tend to 

 diverge from one another, the resulting form of corallum being dendroid 

 or branched. In other cases the new corallites grow up side by side, and 

 in contact with one another, the corallum thus becoming massive or 

 " astraeiform." Composite corals produced by lateral gemmation may 

 have the corallites directly connected with one another, or united by a 

 more or less copious " ccenenchyma" (fig. 133). 



