120 



MANUAL OF ZOOLOGY 



SECT. 



tinuous calcareous tube for each polype. In the red coral 

 of commerce (P"ig. 57), which inhabits the Mediterranean 

 Sea, there is an extremely hard calcareous branched rod 

 which extends as an axis through the ccenosarc. In the 

 black corals (Antipathes and allies) there is a horn-like 

 axis ; and in Gorgonia there 

 is a similar skeleton, some- 

 times partly calcareous, with the 

 addition of numerous spicules. 

 In the sea-pens (Fig. 60) the 

 colony is supported by an un- 

 branched horny axis. Pennatula 

 aculeata lives in deep water in 

 the North Atlantic. 



In the Madrepore corals we 

 have a skeleton of an entirely 

 different type, consisting, in fact, 

 of a more or less cup-like calca- 

 reous structure secreted from the 

 ectoderm of the base and column 

 of the polype. When formed by 

 a solitary polype such a " cup- 

 FIG. sg.-TuWporamusica. Skei- coral" is known as a corallite ; 



eton of entire colony. Natural 



size //.platform. indianOcean. i n the majority of species a large 



(After Cuvier.) 



number sometimes many thou- 

 sands of corallites combine to form a corallum, the skele- 

 ton of an entire coral-colony. 



The structure of a corallite is conveniently illustrated by 

 that of the solitary genus Flabellum (Fig. 61, A, B). It 

 has the form of a short conical cup, much compressed, 

 so as to be oval in section. Its wall or theca is formed 

 of dense stony calcium carbonate, the proximal end pro- 

 duced into a short stalk or peduncle. From the inner 



