PHYLUM COELENTERATA 



191 



formed of a black horn-like material. Moreover it is not meso- 

 dermal, but ectodermal in origin : in close contact with it is an 

 epithelium, from the cells of which it is produced as a cuticular 

 secretion, and this epithelium is formed as an invagination of the 

 base of the colony. In addition to its axis, Gorgonia contains 

 numerous spicules in the mesoderm of the coenosarc. In some 



FIG. 141. Gorgonia verrucosa. A, entire colony; B, portion of the same magnified; 

 coanosarc ; p. polype. (After Koch and Cuvier.) 



of the Gorgonacea the axial skeleton is partly horny, partly 

 calcareous. 



In the Sea-pen (Pennatula, Fig. 134) and its allies the stem of 

 the colony is supported by a horny axis which is unbranched, not 

 extending into the lateral branches. In this case the axis is 

 contained in a closed cavity lined by an epithelium, the origin of 



