592 URINOGENITAL ORGANS CHAP, xn 



mesoderm in the body- wall at the anterior end of the 

 coelome ; these are arranged metamerically and each opens 

 into the coelome by a ciliated funnel (nst). Obviously 

 such tubes are nephridia l (compare p. 340) ; their chief 

 peculiarity is that their outer ends do not open directly on 

 to the exterior, but into a longitudinal tube, the pronephric 

 duct (sg.d), which passes backwards and discharges into the 

 cloaca. It seems probable that this arrangement is to be 

 explained by supposing that the nephridia originally opened 

 externally in to' a longitudinal groove, which, by the apposition 

 of its edges, was converted into a tube. The nephridia 

 of the pronephros open, by their ciliated funnels, into the 

 anterior end of the coelome, into which projects a branch 

 of the aorta on either side ending in a single large glomerulus 

 (p. 146). 



The pronephros soon degenerates, its nephridia losing 

 their connection with the duct (^), but in the mean- 

 time fresh nephridia, developed in a somewhat different 

 manner, appear in the segments posterior to the pronephros 

 and together constitute the mesonephros or Wolffian body 

 (ms. npti), from which the permanent kidney is formed in the 

 lower Craniata. The mesonephric nephridia open at one 

 end into the duct (sg. d. and Figs. 157 and 158), at the other, 

 by ciliated funnels (nst\ into the coelome ; a short distance 

 from the funnel each gives off a blind pouch which dilates 

 at the end and forms a Malpighian capsule (m. c), and 

 a branch from the aorta entering it gives rise to a glomer- 

 ulus. 



In some forms (e.g. dogfish, p. 466) the pronephric duct 

 now becomes divided by a longitudinal partition into two 

 tubes : one retains its connection with part or the whole of 



1 It is doubtful whether these pronephric tubules are to some extent 

 comparable to the nephridia of Amphioxus (p. 424). 



