88 CCELENTERATA. 



the polypes are outside the corallum. In other words, the 

 polypes take no part in the secretion of the corallum; but this 

 is deposited solely by the coenosarc o'r common flesh by which 

 the polypes are connected together. 



Fig. 37. Part of a living stem of A tit if at/if s anguina, of the natural size. 

 (After Dana.) 



The Zoantharia sclerobasica are not known as occurring in 

 either the Palaeozoic or Mesozoic period. They appear for 

 the first time in Tertiary deposits, and the genus Antipathes 

 is represented in strata of Miocene age. 



C. ZOANTHARIA SCLERODERMATA. 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 sclerodermata, in its essen- 

 tial structure resembles a sea-anemone ; but it very often has 

 the power of repeating itself by budding (gemmation) or cleav- 

 age (fission), so that from 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 " cceno- 

 sarc." 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 calcifi- 

 cation 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. 38), 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: i. 

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

 2. The parts secreted by the ccenosarc which unites all the 

 polypes into an organic whole. A compound coral may be 

 theoretically regarded, therefore, as consisting of a greater or 

 less number of simple corals, such as the preceding, united 

 together by a greater or less quantity of calcareous matter 



