212 



GASTROPODA. 



while the right forms a sac (Fig. 101, re) and unites with the ecto- 

 derm to form the efferent duct. In Bythinia, the kidney can at this 

 stage be recognised as a derivative of the posterior part of the peri- 

 cardial sac (Fig. 92 B, re). At a later stage, a process grows out 

 from its postero-ventral part and becomes connected with the ecto- 

 derm of the mantle-cavity, so that the lumen of the kidney now 

 communicates with the latter. In Paludina, the formation of the 

 efferent renal duct (ureter) takes place from the mantle-cavity, which 

 at an earlier stage sank in on the right ventral side. The pallial 



7WL' 



Fig. 98.— J, transverse section through the pericardial region of au embryo of 

 Paludina vivipara at the .stage depicted in Fig. 59 C. B, the kidney of an almost 

 mature Paludina embryo (after v. Bblanger). i'. intestine ; h, rudiment of heart ; 

 / liver; Ih, body-cavitv ; m, stomach; mes, mesodermal tissue; u, definitive, n , 

 abortive kidney'; »", efferent duct of the former; oe, aperture of the kidney into 

 the pericardium ; /-, pericardium ; -y>, pericardial septum (remnant of partitim,- 

 wall). 



depression is prolonged in the direction of each of the kidney-rudi- 

 ments (Fig. 97, na and na'). The branch running towards the right 

 kidney is specially distinct, being longer than that running towards the 

 left rudiment ; the latter, indeed, has no permanent significance on 

 account of the degeneration of this left rudiment. The right branch 

 of the mantle-cavity, however, then fuses with the right kidney, and 

 thus becomes its ureter (Figs. 59 /:. and 98, na). 



