ON THE CCELOM, GENITAL DUCTS, AND NBPHRIDIA. 477 



On the Coelom, Genital Ducts, and Nephridia. 



By 



Edwin S. Ooodi'icli, F.L.S., 

 Assistant to the Linacre Professor of Comparative Anatomy, Oxford. 



With Plates 44 and 45. 



The chief object of this paper is to call attention to a 

 theory of the homology of the coelom which has been gradually 

 gaining ground abroad, but has not, I venture to think, 

 received in this country the notice which it deserves. The 

 theory I refer to is, that the cavity which we know as the 

 coelom in the higher Coelomata is represented by that of the 

 genital follicles in the lower types of that grade. Ever since 

 Hatschek wrote the often-quoted words : ^' Die secundare 

 Leibeshohle verhalt sich wie die Hohle der Geschlechtsdriise 

 der niedrigeren Formen/^ and pointed out that true meso- 

 blastic metamerism is really due to the repetition of the gonads 

 (48), so many favourable new facts have been brought to light, 

 that from a suggestion the statement has become a well- 

 established theory. In a most interesting and suggestive 

 paper on the ancestry of the Annelids, Dr. E. Meyer has set 

 forth the theory in some detail (81). After showing that the 

 coelomic cavities in the Polychaetes are quite comparable in 

 development and function with the genital follicles of the 

 Planarians, he further maintains that the theory throws a 

 flood of light on various otherwise obscure questions, such as 

 the bilateral character of the coelom, its invariable connection 

 with the genital cells, the absence of a truly unpaired prostg- 



