No. I.] GERM-LAYERS IN CLEPSINE. 107 



derivation of the ventral nerve-cord, and my studies have led 

 me to results which contradict the conclusions of both these 

 authors. Although the origin and development of the neph- 

 ridia and the sexual organs have received special attention in 

 recent papers, these questions are still very far from being 

 definitely settled. The precise origin of the nephridia forms 

 just now a question of considerable theoretical importance, — 

 an importance which will be found to be very much increased 

 by the facts to be presented in this paper, and by the parallel 

 results reached by Wilson in his study of Lumbricus. We have 

 long been prepared to believe in the homology of the germ- 

 bands of the Hirudinea and the Oligochseta ; but such a com- 

 plete parallel, both in the mode of origin and in the histological 

 development, as is shown in these two papers, will certainly be 

 a surprise to most embryologists. The observations have been 

 made, independently, on representatives of two different groups 

 of annelids, and they confirm each other in a manner too posi- 

 tive to leave any room for a reasonable doubt of their essential 

 accuracy. Allowing that they are correct, it will be seen that 

 they furnish what we have long stood in need of, — a satisfac- 

 tory basis for the comparative study of the germ-layers in 

 annelids, — and that they give us one more clue to the ancestral 

 history of the vertebrates. 



For the observations recorded in this paper, I had only a 

 single batch of eggs of Clepsine parasitica (?) Say, and a few 

 eggs of C. complanata obtained at Naples. This scanty 

 material did not enable me to carry the investigation beyond 

 the stage in which the concrescence of the germ-bands is nearly 

 completed. I am now able, however, to account for the origin 

 of all the germ-layers, and to give the earlier history of the 

 germ-bands. I have been able to add something to our knowl- 

 edge of the development of the head, and I hope soon to 

 obtain material for a thorough study of this very important part 

 of the subject. The methods employed have been described 

 in the " American Naturalist," Nov., 1885, pp. 1134-1135; and a 

 preliminary notice of the results obtained was published in the 

 " Zoologischer Anzeiger," No. 218, 1886. 



