184 



FOSSIL ACTLNOZOA. 



produced by the calcification, or conversion into carbonate of 

 lime, of the lower portion of a polype similar in structure to 

 an ordinary Sea-anemone. The " theca " of the coral corre- 

 sponds to, and is secreted by, the " column-wall " or general 

 wall of the body of the polype. The " septa," again, corre- 

 spond with the " mesenteries," and, like them, are " primary," 

 " secondary," or " tertiary," according as they reach the centre 

 or fall short of it by a greater or less distance. We must 

 remember, however, that it is only the inferior portion of the. 

 body of the polype which is thus calcified. The tentacular 

 disc and mouth are placed at some distance above the upper 

 margin of the theca, and the digestive sac occupies the calice ; 



Fig. 72. A, Portion of the corallum of Favosiles favosa, of the natural size ; B, Portion of 

 four corallites of Favosites Gothlandica, enlarged, showing the tabulae. 



whilst the whole of the space comprised within the theca is 

 lined by the endoderm, and the whole of its outer surface is 

 covered by the ectoderm. 



Having now considered the general structure of a simple 

 sclerodermic corallum, as produced by a single polype, we 

 must glance for a moment at that of a compound corallum of 

 the same group. Such a corallum is the aggregate skeleton 

 produced by a colony of polypes, each of which is essentially 

 similar to a Sea-anemone in structure, and it varies in size 

 and form according to the characters of the colony by which 

 it is produced. Such a colony (fig. 73) consists of a number 

 of polypes, which may spring directly from one another, or 

 which may be united by a common flesh or " coenosarc," and 

 corresponding differences are found in the resulting corallum. 



