ZOANTHARIA. 



159 



in structure, and the "coral' : it forms is its external shell 

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

 ectoderm. From one polype 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. 

 On this plate a number of radially arranged vertical ridges 

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



5 



FIG. 75. The formation of a coral shell (Asfroides}. 

 After Pfurtscheller. 



sf., stomodaeum ; ;//.?., mesentery; s., calcareous septum ; B., basal plate. 



height they push the ectoderm of the base up before them 

 (see Fig. 75). 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 

 coalescence of septa in the central line a columella or median 

 pillar may be formed. The outer wall of the theca may 



