386 



COELENTERATA ANTHOZOA 



The calieoblasts form, in the first instance, a skeletal plate at 

 the aboral end of the coral embryo, which becomes turned up at 

 the edges to form a shallow saucer or cup. This cup is called 

 the " prototheca." ^ At this stage the body- wall of the living 

 zooid may or may not overflow the edge of the prototheca. In 

 the former case the growth of the rim of the prototheca is 

 brought about by the calieoblasts of an inner and outer layer of 

 epiblast, and the cup is then called the " theca." In the latter 



Fig. 168. — Diagram of a vertical 

 section of a young Caryophyllia, 

 showing the septa (S) covered 

 with endoderm projecting into 

 the coelenteric cavity. M, 

 mouth ; St, stomodaeum. (After 

 G. vou Koch.) 



Fig. 169. — A young Caryophyllia, viewed from 

 above, showing the tentacles (t) and the 

 stomodaeum (St). The letter m points to a 

 space between a pair of mesenteries, and 

 the darker shading in this place shows a 

 septum projecting radially from the wall of 

 the theca. (After G. von Koch.) 



case, the growth of the rim of the prototheca is continued by the 

 calieoblasts of one layer of epiblast only, and it is called the 

 " epitheca " (Flahellum). With the continued growth of the theca 

 the tissues that have overflowed — the " episarc " — retreat from 

 the base, and in doing so the ectoderm of the edge and, to 

 some extent, the outer side of the episarc secrete a layer of 

 epitheca which becomes more or less adherent to the theca. 

 Thus the cup may have a double wall, the theca and the epi- 

 theca {Caryophyllia). 



With the growth of the theca and epitheca a certain number 

 of radially disposed laminae of lime rise from the walls and 

 grow centripetally. These are the " septa." Additional ridges on 

 ^ H. M. Bernard, Ann. Mag. Nat. Hist. (7) xiii. 1904, p. 1. 



