598 DR. R. V. ERLANGER. 
therefore, distinguish an upper and a lower nook of the 
mantle cavity on either side (Msr., Msl., Mir., Mil.). The anus 
lies at the end of a well-defined papilla which occupies the 
centre of the cross which divides the mantle cavity in the way 
which has just been described. On the ventral side of the 
anus, just above the floor of the mantle cavity (Ma.), a project- 
ing ridge parallel to the mantle cavity may be seen. This 
ridge is formed by the basibranchial sinus (bas.). The ridge 
itself bears the two papillz on which the external apertures of 
the two kidneys lie. The papilla of the left kidney (Pr.) is 
larger, has a slit-like opening (X7.), and is situated anteriorly 
on the right edge of the basibranchiai ridge. The papilla of 
the left kidney (P/.) is much smaller, and lies further back in 
the left inferior nook of the mantle cavity. Its aperture is 
nearly round, and in most cases very difficult to find in dissec- 
tions. In the preparation just described, the ctenidia which 
are cut transversely display perfectly the double feathered 
condition characteristic for the forms with two equally deve- 
loped gills. The relations of the ctenidia to the auricles 
(aur.), the ventricle, the nipples of the anus, and the kidneys 
are shown in fig. 10, which is drawn from Fiss. maxima, in 
which the pericardium and mantle cavity have been opened 
ventrally. 
At the base of the papilla of the right kidney (Pr.), between 
this and the pericardium, that is to say, in the hinder wall of 
the basibranchial sinus, two openings can be seen. ‘The one 
lying towards the middle line, distinguished by a brownish 
colour, is the beginning of the left kidney running into its 
papilla (X7r.); while the other one, lying outside of the first, is 
the genital duct (Gd.). Both kidney and genital duct have 
been cut through transversely in the course of the dissection. 
The same thing magnified ten times is shown in fig. 18. A 
bristle has been introduced into the genital duct, and is seen 
to issue out of the sectionised right kidney—a proof that the 
genital duct opens into the papilla of the right kidney. The 
left kidney can be easily detected in Fiss. reticulata, 
maxima, gibberula, and greca; it is a remarkable fact 
