C(ELENTERATA. 157 



are found calcareous radiating plates known as septa. 

 Thus the coral lies, morphologically speaking, outside 

 the body of the polyp, although the septa project into 

 the enteron. The edges of the septa are seen as 

 characteristic ridges on the inner rim of the calcareous 

 cup, or theca, which is left when the polyp has 



Fig, 34. — An Englisli coral, Cai*i/oj)7iyIlia cyafTiws. 



decayed. These are the so-called Sclerobasic corals ; 

 the reef-building corals are of this kind. 



In the Alcyonaria, on the contrary, the skeleton is 

 found in the mesogloea, though it is considered possible 

 that the cells which secrete it may have migrated there 

 from the ectoderm. In Alcyonium it consists of free 

 calcareous spicules (needles) ; in Tubipora these unite 

 to form the tubes ; in other Alcyonaria loose spicule^ 



