PLATYHELMIA 59 



expanded as to form a single cavity a cavity containing a 

 watery fluid, across which the ectoderm and the endoderm 

 are connected only by thin threads of the original mesoderm. 

 The space, like the mesenchyme of the flat worms, is invaded 

 by protonephridia derived by invagination of the ectoderm, 

 and they end blindly in flame cells or, as they are now 

 generally termed, solenocytes. This primitive body cavity is 

 called the primary body cavity, or schizocoel, and the cells 

 which occur within it are derived from both ectoderm and 

 endoderm. 



A secondary body cavity, or coelom, is developed in the 

 remaining groups of animals typically, and they may be thus 

 described as Coelomata. It is derived from the endoderm, 

 and the cells form a cavity distinct from the schizocoel. An 

 anterior schizocoel and a posterior coelom are thus present in 

 the early Coelomata. The appearance of a coelom is correlated 

 with the formation of blood and vessels by the cells of the 

 schizocoel, which thus preserve their primitive function of 

 circulating the metabolic products. It is obvious that an 

 expansion of the coelom can take place only at the expense of 

 the schizocoel, and vice versa. The coelom in some undoubted 

 Coelomata may be suppressed. 



The coelom is, however, not a new formation. In the 

 Platyhelmia the mesoderm is resolved into a tubular system, 

 concerned with the development and the discharge of the 

 sex cells, and a primary body cavity. The Platyhelmia, like 

 the Rotifera, have, in other words, a gonocoel and a schizocoel. 

 The gonocoel in the Eotifera is derived from the endo- 

 derm, and may, indeed, be said to have this origin in the 

 Platyhelmia. The coelom is the expanded gonocoel, and, 

 invading the territory of the schizocoel, it takes over to some 

 extent the excretory function. The protonephridia either 

 establish an opening into the coelom, or are suppressed and 

 special ducts developed from the coelom which are called 

 coelomiducts. 



A coelom emerges in a group of animals which are all so 

 intimately connected at their roots, although divergent in 

 their stems and branches, that it is difficult to separate them 



