210 



ONYCHOPHORA. 



for the formation of the muscles and the connective tissue. The boundaries 

 of the pseudocode are thus formed, the latter arising on the whole in the way 

 already described by the separation from one another of the primary germ- 

 layers and the formation of cavities in the massive mesodermal tissue, the 

 remaining spaces belonging to the primitive segments naturally being added 

 as their boundary walls break up. The continuity of the lining epithelium 

 is only retained in the middle (ventral) portion (II) of the primitive segment. 



This yields the funnel of the nephridium (Fig. 

 104 A-C, II), which, according to this account, 

 as also according to the statements of Balfour 

 and Gaffron, is open towards the adult body- 

 cavity. This funnel, of mesodermal origin, is 

 joined by a ventral invagination of the ectoderm, 

 which proceeds from the base of the foot and 

 grows out like a tube (Fig. 104 A-C, nc). 



Thus while Sedgwick derives the whole of 

 the nephridium from the mesoderm, v. Kennel 

 traces the origin and the greater part of the 

 nephridium to the ectoderm. In our previous 

 description we followed the accounts of Balfour 

 and Gaffron, because, from researches made by 

 Bergh * in connection with the Annelida, it 

 has become highly probable that the nephridia 

 in those animals are formed entirely from the 

 mesoderm. 



The derivation of the nephridial canal from 

 the ectoderm must also affect v. Kennel's view 

 as to the formation of the genital organs, since 

 he too acknowledged the efferent genital ducts 

 as transformed nephridia. Not only the terminal 

 portion, but the whole of the efferent ducts is 

 therefore of ectodermal origin ; only a short 

 piece, connecting the ectodermal uteri and vasa 

 deferentia with the genital glands is yielded by the 

 mesodermal nephridial funnel (Fig. 105 A-C, ml). 

 From this, in the female, are produced the acces- 

 sory structures of the uterus. A glance at Fig. 

 105 A-C makes this clear. The female in 

 P. novae-zealandiae has a paired receptaculum 

 seminis, and in the American species a paired 

 receptaculum ovorum as well, which opens 

 between the former and the ovary, and close 

 to the latter, into the uterus. These accessory 

 structures are not found in the female in 

 P. cajiensis. 



The paired receptaculum seminis arises 

 as follows : — Each of the two uteri makes 

 a sharp bend behind the ovary, so that 



Fig. 104. — Portions of sections 

 through embryos of P. Ed 

 wardsii at various stages (dia 

 grammatic, after v. Kennel 

 from Lang's Text-book of Comp 

 Anat.). d, intestine ; I, foot 

 Ih, body-cavity ; m, mesodermal 

 tissue ; n, ventral nerve-trunk 

 nc, nephridial canal ; /, II, III, 

 the three spaces of the primi 

 tive segment-cavity, // repre 

 senting the rudiment of the 

 funnel. 



* R. S. Bergh, Neue Beitrage zur Embryologie der Anneliden, Theil i., 

 Zcitsclir. f. wiss. Zool, Bd. h, 1890 ; cf. also Vol. i., p. 297. 



