574 URINOGENITAL ORGANS ch. xr 



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

 such tubes are nephridia (compare p. 331); 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 into a longitudinal groove, which, by the apposition 

 of its edges, was converted into a tube. All the nephridia 

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

 narrow anterior end of the coelome, into which projects a 

 branch of the aorta ending in a single large glomerulus 

 (p. 146). 



The pronephros soon degenerates, its nephridia losing 

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

 time fresh nephridia appear in the segments posterior 

 to the pronephros and together constitute the mesonephros 

 or Wolffian body (ins. nph), from which the permanent 

 kidney is formed in most of the lower Craniata {e.g. 

 frog). The mesonephric nephridia open at one end into 

 the duct {sg. d), at the other, by ciUated funnels {nst), into 

 the ccelome ; 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 glomerulus. 



In some forms {e.g. Dogfish, p. 450) the pronephric 

 duct now becomes divided by a longitudinal partition into 

 two tubes : one retains its connection with the mesonephros 

 and is now known as the mesonephric or Wolffian duct 

 (C, ins. n. d) : the other, or Milllerian duct {p. n. d), has no 

 connection with the nephridia, but opens into the coelome 

 in the region of the vanishing pronephros, and assumes the 

 functions of an oviduct in the female. In some Craniata the 



