190 



THE INVERTEBRATA 



which encloses them, and in these are formed vertical walls which 

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

 wall septa (Fig. 142 A). The latter are formed in spaces between the 



Fig. 142. 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. first formed part of 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. 



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

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

 tapering basal portion is solid but which has a shallow apical de- 



