ON THE PAIRED NEPHRIDIA OF PROSOBRANCHS. 607 
remaining kidney of Paludina, and opens into the mantle 
cavity ; the other one (left according to Bouvier) has no ex- 
ternal opening, but communicates with the other renal organ. 
Whilst the first renal organ is formed of lamelle, the roof of 
the second one is coated with a vascular web, which constitutes 
the renal tissue. 
The vascular irrigation of the first kidney is the same as in 
the renal organ of Lamellibranchs, as the venous blood which 
has passed through it goes to the gills before reaching the 
heart, whilst the venous blood of the second flows directly into 
the auricle. 
“ The two renal organs of Ampullaria are, therefore 
(says Bouvier), placed exactly in the same conditions as those 
of Haliotis, according to Wegmann’s description, the second 
kidney performing the same part in the circulation as the only 
kidney of Monotocard Prosobranchs.” 
Bouvier, therefore, argues that the first kidney is homo- 
logous with the right kidney, the second with the left kidney 
of Diotocards. 
I myself hold the opposite view on the same subject. 
Bouvier’s most valuable argument is derived from the circula- 
tion in the renal organs. Perrier (23), however, has shown 
that, as we proceed higher in the series of Prosobranchs, the 
circulation in the kidney undergoes very considerable changes. 
The quantity of blood passing through the right kidney before 
going to the gills decreases more and more as we proceed 
from Haliotis to Trochus and Turbo. In Monotocards 
the only remaining kidney has developed an entirely new and 
different system of blood-irrigation, and its circulation has 
grown quite independent of the general one. His argument, 
therefore, turns out to be much less important than it seemed 
at first. 
The position of the kidneys, as far as I can make out by 
examining the figure given by Bouvier of a dissected Ampul- 
laria, would rather tend to prove that the large vascular sac 
lies more to the right, the smaller lamellar organ to the left 
of the rectum and the body axis. The size and shape of both 
