88 INVERTEBRATE ANIMALS. 



The members of this group all possess the power of secreting 

 carbonate of lime within their tissues, so as to form a more 

 or less continuous skeleton or corallum. Froni the fact that 

 this corallum is secreted by the inner layer of the polypes, 

 and is therefore truly within the body, it is said to be " sclero- 

 dermic," in opposition to the kind of coral produced by other 

 forms (such as the red coral), where the skeleton is secreted 

 by the outer layer of the polypes, and is therefore outside 

 them. In this latter case the coral is said to be " sclerobasic." 

 (For illustrations of these different kinds of corals, see Fig. 

 29.) In the typical form of sclerodermic coral, the skeleton is 

 in the form of a conical cup, the upper part of which is hol- 

 low. The lower part is divided into a series of compartments 

 by vertical plates, which are called the " septa," and which 

 correspond to the mesenteries of the living animal. Some- 

 times the space contained within the walls of the cup cr 

 " corallite" is broken up by horizontal plates called "tabulas;" 

 but, when these are present, there are generally no septa. In 

 the form of coral just described we have a single corallite, 

 produced by one polype, and this simple condition may 

 be maintained throughout life. In the great majority of 

 cases, however, the polypes bud, so as to form a colony, all 

 bound together by a common flesh or ccenosarc. When such 

 a colony, therefore, produces a sclerodermic coral, in place 

 of a single corallite, we have a composite skeleton composed 

 of a number of little cups or corallites, each of which was 

 produced by one polype, and all of which are united by means 

 of a common calcareous basis secreted by the ccenosarc (Fig. 

 29, a). 



In accordance with their mode of formation, an ordinary 

 compound sclerodermic coral may be distinguished from a 

 sclerobasic coral by the fact that it would show a number of 

 little cups in which the polypes were contained, whereas these 

 cups would be absent in the latter. In accordance, also, with 

 the fundamental character of the order Zoantharia, the corals 

 of the present group always show septa which are some mul- 

 tiple of five or six. 



When it is understood that compound corals, such as we have been 

 speaking of, are produced by the combined efforts of a number of polypes, 

 essentially the same in structure as our ordinary sea-anemones, it is readily 

 intelligible that under favorable circumstances large masses of coral may be 

 produced in this way. When these masses attain such a size as to be of 

 geographical importance, they are spoken of as " coral-reefs," and the phe- 

 nomena exhibited by these are of such interest as to demand some notice. 

 The coral-producing polypes require for their existence that the average 



