ZOAXTHARIA. 179 



part in the secretion of the corallum ; but this is deposited 

 solely by the coenosarc or common flesh by which the polypes 

 are connected together. 



The corallum is sometimes simple and unbranched, but 

 is more commonly branched and plant-like, and its surface, 

 though occasionally smooth, is usually covered with minute 

 spines, being thus readily distinguished from the grooved and 

 striated sclerobasis of the Gorgonidae. 



The Zoantharia sclerobasica are not known as occurring in 

 either the Palaeozoic or Mesozoic period. They appear for 

 the first time in the Miocene Tertiary (Leiopathes vetusta). 



C. ZOANTHARIA SCLERODERMATA (HEXACORALLA). This 

 group includes most of the so-called " corals," and is of very 

 high geological importance. All the members of this group 

 secrete a skeleton or " corallum," and this is necessarily the 

 only part of the animal with which the palaeontologist has to 

 deal ; so that it becomes necessary to enter into its structure 

 at some length. The animal itself in the Zoantharia sclero- 

 dermata, in its essential structure, resembles a Sea-anemone ; 

 but it very often has the power of repeating itself by budding 

 (gemmation) or cleavage (fission), so thatfrbm a simple it 

 becomes a compound organism. It may therefore consist 

 of a single " polype," or of many similar polypes united by 

 a common flesh or " coenosarc." The corallum is what is 

 called " sclerodermic," its essential peculiarity being that it 

 is secreted by the polype or polypes. The sclerodermic 

 coral, in fact, is an actual calcification of part of the tissues 

 of the polype. When, therefore, we have a simple coral, 

 produced by a simple member of this group (as in fig. 69), 

 we have clearly to do with nothing but skeletal structures 

 produced in the interior of the polype itself. When, on the 

 other hand, we have a compound sclerodermic coral to deal 

 with, we have usually more than this. We have, namely ; 

 two parts or elements of the coral to consider : 1 . The parts 

 of the coral secreted by each individual polype ; and, 2. The 

 parts secreted by the coenosarc which unites all the polypes 

 into an organic whole. A compound coral may be theoreti- 

 cally regarded, therefore, as consisting of a_greater or less 

 number of jmnple_jcorals, such as the preceding, united 



