ZOANTHARIA 167 



rise from the plate ; the circular wall is called the theca and the radial 

 walls septa (Fig. 138 A). The latter are formed in spaces between the 

 mesenteries. The continued secretion by such a form as the English 



pot ^% 



pol. % 



Fig. 138. Skeleton formation in the Zoantharia. A, Oral view of a young 

 coral polyp with the beginning of the skeleton seen through the transparent 

 tissues. B, Vertical section through a later stage. C, Development of a colony 

 showing budding from the extrathecal zone. D, Division of a polyp, pol. 

 polyp before division; pol.' polyp after division and subsequent growth: 

 skeleton of pol. shown in black (as in earlier diagrams) and that of pol.' by 

 stippling ; th. theca ; sep. septum ;ex.th. extrathecal portion of polyp or colony. 



E, Lophohelia. Skeleton of colony, soft parts indicated by dotting, pol. polyp ; 

 pol.' polyp about to divide; th. theca with septa indicated; cch. coenenchyme. 



F, Astroides. After van Koch. Tangential section of young form fixed on 

 cork (ck.). ect. ectoderm; end. endoderm; cal. granular secretion of calcium 

 carbonate forming the basal disc; mes. mesentery; sep. septum. 



solitary coral Caryophyllia produces a cup of limestone, of which the 

 tapering basal portion is solid but which has a shallow apical depression, 

 which is traversed by the radiating vertical septa and contains in the 



