14 THE SUB-KINGDOM CCELENTEEATA. 



remains occur, to reappear in every stratified de- 

 posit of importance intervening between that 

 epoch and the present day. 



2. Classes. — Two leading modifications of 

 structure may be traced among Coslenterate ani- 

 mals, which admit, therefore, of being arranged 

 under two principal groups or classes, the Hydro- 

 zoa and the Actinozoa. 



In the Hydrozoa, the wall of the digestive 

 apparatus is not separated from that of the somatic 

 cavity, and the reproductive organs are external. 



In the Actinozoa, the wall of the digestive sac 

 is separated from that of the somatic cavity by an 

 intervening space, sub-divided into chambers by a 

 series of vertical partitions, on the faces of which 

 the reproductive organs are situated. 



In order to understand these relations aright, 

 it will be necessary to contemplate, from a general 

 point of view, the development of the Coelente- 

 rata. 



The scanty knowledge which we possess of the 

 life-history of the Protozoa has, in previous pages, 

 been sufficiently pointed out. From their known 

 simplicity of structure, it may, indeed, be conjec- 

 tured, that the changes which they undergo during 

 the course of development must be comparatively 

 slight. As already shown, the very existence of 

 reproductive elements has yet to be ascertained in 

 by far the greater number of the members of this 

 sub-kingdom. 



In the other four departments, true reproduc- 

 tion, by contact of ova and spermatozoa, univer- 

 sally occurs. Within the nutrient mass, or ' yolk, 



