170 MOLLUSCA. 
placed on the side next the tail, and supplied by the two 
systemic veins which collect the aérated bleod from the 
branchiz on each side. There is a simple aorta arising from 
the opposite side of the heart. The organs of generation 
appear to be similar to those of the preceding class; but 
they have not as yet been minutely examined. The exist- 
ence of eyes is not satisfactorily determined. The animals 
of this genus appear to be inhabitants of the tropical seas. 
Cuvier has given descriptions and figures of three species, 
which differ remarkably from one another in the protuber- 
ances of the cleak. 
Genus ApiystA.—Branchie with a corneous lid. The 
body of the Aplysia is ovate, acuminated behind, and pro- 
duced before to form a neck. The foot is narrower than 
the body. In the middle of the back is a corneous plate 
inclosed in a bag in the skin, and on each side, and be- 
hind, there is a fold by which this part may be concealed. 
The head is slightly emarginate, with a feeler on each side. 
The superior feelers are situate on the neck. In front of 
each of these is a small black point or eye. The branchie 
are situate underneath the dorsal plate, on the right side, 
and exhibit a complicated plumose ridge, capable of expan- 
sion beyond the edge of the plate. The anus is situate im- 
mediately behind the branchize, and before these is the ori- 
fice of generation, from which proceeds a groove along the 
neck to the inferior base of the fore feeler, on the right side, 
where there is an opening for the penis. Within the longi- 
tudinal lips there are two smooth, corneous plates, the sub- 
stitutes for jaws; the tongue is rough, as in many of the 
other gasteropoda. ‘The gullet is short, and suddenly ex- 
pands into a large subspiral crop, with membranaceous walls. 
To this, a gizzard with muscular walls succeeds, the interior 
