198 



PHYLUM CGELENTERA 



coalescence of septa in the central line a 

 columella or median pillar may be formed. 

 The outer wall of the theca may bear 

 vertical ridges or costae, and these may 

 be connected with neighbouring costae 

 of other polyps by horizontal shelves 

 or dissepiments. Both septa and costae 

 correspond to intermesenteric spaces. 



Antipatharians. — Usually arborescent, some- 

 times whip-like colonies, of wide distribution, 

 often called " black corals." A spinose hollow 



MMWis 



tmtft 



I. 



M. \J 



Fig. 104. — Diagrams of Types of Alcyonaria. — After Hickson. 



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



IV. of Stelechotokea, 



horny axis is covered with coenenchyma and regularly arranged 

 polyps, without any trace of spicules. A polyp is usually oval 

 in'section, with its long diameter in the line of the axis, and its gullet 



