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. Astr@a and Madrepora and Lophohelia 
(North Sea), but there are solitary forms such as Pungia 
and Caryophyllia (British). 
The first part of the “shell” to be formed is the dasal 
plate between the ectoderm of the base and the substratum. 
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Fic. 82.—The formation of a coral shell (Astroides).— 
After Pfurtscheller. 
st., Stomodzeum ; 7zs., mesentery ; s., calcareous septum ; &., 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 ¢heca 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 efztheca is 
formed outside of and concentric with the theca. By the 
coalescence of septa in the central line a colume//a or median 
