ALCYONARIA. 



161 



of a single primary polype which gives off 

 solenia hearing numerous secondary polypes, 

 e.g. Pennatula, a type of Stelechotokea (cf. 

 Fig. 76, IV.). 



An altogether aberrant type is represented 

 by the blue coral (Heliopora) and its extinct 

 relatives (Heliolites , etc.). 



Before we proceed to the systematic survey, 

 we may contrast the essential structural features 

 of the four classes of Coelentera. 



I. In the Hydromedusce there is no inturned 

 ectodermic gullet or stomodaeum ; there are no 



V 



\J 



FIG. 76. Diagrams of Types of Alcyonaria. After Hickson. 



Types of Alcyonaria : I. Of Stolonifera ; II. of Alcyonacea ; III. of Axifera ; 



IV. of Stelechotokea. 



partitions or mesenteries ; there are no special digestive organs ; in 

 the body wall the ectodermic muscles are mostly longitudinal and the 



ii 



