594 DR. R. V. ERLANGER. 
rectum (R.), and above the mantle cavity (M.). This sinus 
is morphologically equivalent to the basibranchial sinus exist- 
ing in Haliotis and in Fissurella-like forms, where we shall 
meet with it again. Perrier (23) and Bernard (1) have shown 
that the blood coming from the right kidney, the head, and the 
epipodium is collected here, and hence is sent to the ctenidium 
or ctenidia. 
The vein (v.) which in Trochus runs into the afferent vein 
of the ctenidium is seen to the left of the rectum, dorsally from 
the left kidney (N/.) or papillary sac. This organ has the 
closest resemblance to the corresponding one of Haliotis, and 
shows a great number of the characteristic papille. 
The right kidney (Nv.) is to be seen on the same section 
to the right of the basibranchial sinus. The section only 
shows the part which acts as a reservoir for the urinary secre- 
tion, and has been called urine chamber (Urinkammer) by 
B. Haller (13). 
Projecting into the ventral wall of the papillary sac can 
be seen the transverse section of a duct which lies between the 
mantle cavity and the papillary sac or left kidney. In fig. 3 
(Y.) it is seen to open into the left kidney ; two or three sec- 
tions further back this communication is entirely obliterated (C, 
fig.8). Proceeding still further backwards, the duct in question 
can again be seen to open into a space lying ventrally to the 
basibranchial sinus (fig. 12), and which, as another section 
still further back clearly shows, is a ventral and anterior pro- 
longation of the pericardium (Pc.). In fig. 12 two distinct 
portions of this cavity may be seen (Pc. and Pc.), the one 
already described and opening into the reno-pericardiac duct 
(y.), the other separated from the first by the basibranchial 
sinus (bas.) lying immediately below the rectum, which is 
already enveloped by the ventricle (V.). Fig. 7 shows that 
further backwards the two portions unite (Pc.), and that now 
the basibranchial sinus (das.) is divided by the pericardium 
into two separate halves. 
By comparing the transverse series with horizontal ones in 
which the reno-pericardiac duct can be readily found, and with 
