ON THE PAIRED NEPHRIDIA OF PROSOBRANCHS. 605 
The genital products always pass through the right renal 
organ, either by bursting of the gonad through the walls 
of the right kidney, as in Patella and Trochus,! or being 
admitted through a kind of valve (Haliotis), or trans- 
ported to the right renal papilla by a special genital duct 
(Fissurella). 
The inference to be drawn from these statements is that, 
according to the view first expressed by Ray Lankester (19), 
the only remaining nephridium of most Prosobranchs corre- 
sponds to the actual left kidney of forms possessing two renal 
organs. In my paper on the development of Paludina (8) I 
have stated that this view was supported by embryological 
evidence, the actual nephridium of Paludina being situated 
before the torsion to the right of the anus, while the rudi- 
ment of the actual right kidney lies to the left of the rectum 
before the torsion takes place. I have further shown that, as 
Sarasin® had already stated, the actual nephridium in By- 
thinia tentaculata was, before the torsion, situated to 
the right of the anus, as must be the case in all leiotropic 
forms. In Planorbis, on the contrary, which is a dexio- 
tropic species of fresh-water Pulmonates, the actual right 
nephridium is, before the torsion, situated to the left of 
the rectum (10). 
These facts, compared with the fact that the genital gland 
in the above-mentioned species of Zygobranchs and Cyclo- 
branchs, or, according to the new French terminology (3), 
Diotocards and Heterocards, opens into the right kidney, 
and that in Paludina the duct of the rudimentary right kidney 
becomes the genital duct, show clearly— 
1. That the only remaining kidney in most Proso- 
branchs is the actual left one. 
2. That the actual right kidney has disappeared or become 
' In Turbo and probably also in Trochus, the genital gland has a separate 
opening into the mantle cavity (v. Jhering (16), and “Sur les relations 
naturelles des Cochlides et des Ichnopodes,” in ‘ Bulletin scientifique de la 
France et de la Belgique,’ xxiii, ] Partie, 1891, pp. 148—257, pl. iv—vi). 
2 Sarasin, P., “ Entwickelungsgeschichte der Bythinia tentaculata,” 
in ‘Arb, z. Inst. Wurzburg,’ Bd. vi, 1882, pp. 1—68, Taf. i—vii. 
