Manchester Memoirs, Vol. Ix. (191 6). 9 



which build up these great colonial masses are alike in 

 each genus and exhibit an almost perfect radial symmetry. 

 Similarly with the Alcyonaria in which group we find an 

 even greater variety of shape and structure in the colony 

 as a whole there is a most remarkable similarity in the 

 anatomy as well as in the general symmetry of the con- 

 stituent polyps. 



Now if we consider the general structure of these 

 colonies apart from their shape or symmetry, we find that 

 in the first place there is a certain, general correspondence 

 between members of the same zoological groups as regards 

 the actual constitution of the skeletal elements. Thus in 

 all the Madreporaria the skeleton is composed of a crystal- 

 line calcium carbonate, in nearly all the Alcyonaria the 

 tissues are supported by defined and shaped spicules of 

 the same substance, in nearly all the Hydroid zoophytes 

 the skeleton is formed by secretions of chitin or keratin, 

 so that in a general way the zoologist can determine the 

 zoological affinities of a given specimen by the structure 

 of these common colonial structures. In the second place 

 the details of the structure in the colonial skeletons 

 frequently correspond with the zoological affinities of the 

 individual polyps. Thus the septa of the Madreporaria, the 

 axis of certain Alcyonaria, the thecae of the Calyptoblastea, 

 the minute tabulate pores of the Milleporina, the styles of 

 the Stylasterina, and many other characters can be used 

 with confidence as guides to zoological affinity. 



But when we take the shape, the general symmetry, 

 or the method of branching as characters of a sedentary 

 colony, we find as a general rule that they are no safe 

 guides to zoological affinity. The various forms of pal- 

 mate, plicate, ramose, sub-ramose, foliaceous, and many 

 other types of branching occur over and over again in 

 different genera of Corals, and it is constantly found that 



