ZOANTHARIA. 185 



In the former instance, as previously remarked, the compound 

 corallum consists of an assemblage of separate " corallites," 

 as the skeletons of the individual polypes are called, these 

 beino- united with one another directly and in various ways. 



Fig. 73. Astrcea pallida, a compound sclerodermic coral, in its living condition. 

 (After Dana.) 



Tn the latter instance the corallum consists of a number of 

 " eorallites," and of a common calcareous basis or tissue, which 

 unites the various corallites into a whole, is secreted by the 

 coenosarc, and is known as the " ccenenchyma." 



The compound coralla are, of course, primitively simple, and they 

 become composite either by budding or by cleavage of the original 

 polype. The following are the principal methods in which this increase 

 is effected ; and in considering this subject briefly, it will be as well to 

 take into account not only the Zoantharia sclerodermata, but also the 

 Rugosa, the modes of increase in the two groups being very similar : 



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

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

 and the circle of tentacles, and these buds on becoming perfect corallites 

 may repeat the process. This is one of the commonest modes of growth 

 amongst the recent corals, and it gives rise chiefly to dendroid or tree- 

 like corals. 



2. Basal gemmation. In this method the original polype gives forth 

 from its base a rudimentary ccenosarc, from w r hich new buds are thrown 

 up, and which may have the form of root-like prolongations or of a con- 

 tinuous horizontal expansion. The resulting coralla are usually massive 

 or incrusting, and the youngest corallites are, of course, those placed on 

 the periphery of the colony. 



