PHYLUM OELENTERATA 



121 



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. 57) the colony is supported 

 by an unbranched horny axis. 



In the Madrepore corals we have a skeleton of an entirely 

 different type, consisting, in fact, of a more or less cup-like 



FIG. 57. Pennatula SUlcata. Entire colony. (After Koelliker.) 



calcareous structure secreted from the ectoderm of the base 

 and column of the polype. When formed by a solitary 

 polype, such a " cup-coral " is known as a corallite ; in the 

 majority of species a large number sometimes many 

 thousands of corallites combine to form a corallum, the 

 skeleton of an entire coral-colony. 



