ZOANTHARIA, 



165 



In a general way a Madrepore polyp is like a sea-anemone 

 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 polyp others usually arise by budding 

 or by division, e.g. Astrcza 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. 82. The formation of a coral shell {Astroides). 



After Pfurtscheller. 

 st., Stomodaeum ; MS., mesentery ; s., calcareous septum ; B. t basal plate. 



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. 82). 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 



I 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 



