ZOANTHARIA 



197 



rather than its skeleton. It is altogether a product of the 

 ectoderm. From one polyp others usually arise by budding 

 or by division, e.g. Astrcea and Madrepora and Lophohelia 

 (North Sea), but there are solitary forms such as Fungia 

 and Caryophyllia (British). 



The first part of the " shell " to be formed is the basal 

 plate between the ectoderm of the base and the substratum. 



FiG. 103. — Structure of Antipatharians. 



1. A group of polyps — M., mouth ; t., tentacles. 



2. Axis without polyps and ccenenchyma, covered with spines 



[Sp.). 



3. Vertical section of a polyp — ^., axis; ^, tentacle; g., gullet; 



m., mesentery ; 0., ovary ; m.f., mesenteric filaments. 



4. Cross-section of a polyp — EC, ectoderm; M., mesogloea ; 



EN., endoderm ; G., gullet ; MS., mesenteries. 



On this plate a number of radially arranged vertical ridges 

 (septa or cnemes) are then formed, and as they grow in 

 height they push the ectoderm of the base up before them 

 (see Fig. 102). An external wall or theca is then formed, 

 partly by the fusion of the outer margins of the septa and 

 partly by a circular upgrowth from the basal plate. This 

 theca pushes the body wall before it, as the septa pushed 

 the base. Sometimes a second external wall or epitheca is 

 formed outside of and concentric with the theca. By the 



