1 62 



MANUAL OF ZOOLOGY. 



developed (fig. 76, D), they are transverse plates, which extend 

 completely across the visceral chamber, and divide it into a 

 series of storeys placed one above the other, the only living 



Fig. 76. A, Portion of the corallum of Pocillopora aspera, var. lata, Verrill, of the 

 natural size. B, Part of the surface of same, enlarged. C, Section of the corallites of 

 the same, showing the columella, enlarged. D, Vertical section of the same, en- 

 larged, showing tabulae. (After Dana.) 



portion of the coral being above the last -formed tabula. 

 Tabulae are found in various of the Zoantharia sclerodermata, 

 in some of the Alcyonaria^ and in a great many of the Rugosa. 

 The above gives the general structure of a typical simple 

 sclerodermic corallum, as secreted by a single polype. A 

 compound sclerodermic corallum is the aggregate skeleton pro- 

 duced by a colony of such polypes, and varies in form and 

 size according to the characters of the colony by which it is 

 produced. In general, such a colony consists (fig. 77) of a 

 number of polypes, which may spring directly from one an- 

 other, 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 consists of an assem- 

 blage 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 corallum con- 

 sists of a number of " corallites," and of a common calcareous 

 basis or tissue, which unites the various corallites into a whole, 

 is secreted by the ccenosarc, and is known as the "coenen- 

 chyma." 



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

 come composite either by budding or by cleavage of^the ^riginal polype. 

 The following are trie 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 : (i.) Lateral or parietal 



