ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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liomologuo of the singlo orifice of other Molluscs ; the other ducts appear 
to be secondary or cenogenetic, aud arc probably due to the many 
points of contact between the pericardium and the kidney. As the kidney 
of Elysia is a single unpaired nepliridium and not derived from tho 
fusion of several nephridia, as is probably the case in flat Worms, this 
multiplicity of ducts is no support to the view that tho Mollusca are 
allied to the Platyhelminthes. 
The gonad is much broken up and scattered in the posterior part of 
the body ; it is made up of globular acini which are entirely hermaphro- 
dite ; at the same time, the hermaphroditism is protandric. Of the 
three genital ducts the most ventral is the spermoviduct, the median is 
the duct of the so-called prostate, and the most dorsal is the duct of the 
tubular gland, which extends through the whole of the body. The 
true genital duct divides into two branches, of which the left is oviducal, 
and has on its course a uterine enlargement ; the uterus receives the 
duct of the large copulatory pouch, a second branch from which opens 
into the oviduct; the right branch is connected with the penis. In addition 
to the orifices of these two ducts there is a third which is connected by a 
fine duct with the uterus, and has a vaginal function. It is remarkable 
that this third orifice and its canal are developed later than the two 
other genital openings ; Limapontia exhibits a similar bifurcation of 
the female ducts. 
Nervous System of Nerita and Navicella.* — From a study of the 
nervous system of these Molluscs, and a comparison thereof with that of 
Turbo , M. L. Boutan is led to conclude that there are no orthoneurous 
Aspidobranchs, and that the division into chiastoneurous and ortho- 
neurous Aspidobranchs is based on an incomplete study of the nervous 
system. In Nerita we find the same ganglion as in other Aspidobranchs, 
and the homology between the different parts of the nervous system is 
complete. The cerebral ganglia are connected with one another by a 
commissure which passes below the digestive tube, and is independent 
of the stomatogastric ganglia. The pedal ganglia have the form of a 
ganglionic chain, but only supply the foot proper. The two first ganglia 
of the visceral centre innervate the mantle and the muscles of the shell ; 
they are connected with the rest of the visceral system by two crossing 
commissures, which form a figure of 8 before passing to the ganglia 
situated above the digestive tract. The fifth ganglion of the visceral or 
asymmetrical centre is represented, in its usual position, by a ganglion 
which sends out branches to the anterior part of the digestive tube. 
The special character of the nervous system of Nerita is due to the 
special importance of the pallial nerves, and to the fact that one enters 
into close relations with the left gill. This pallio-branchial nerve is 
prolonged into the respiratory organ, which is therefore innervated by 
the first asymmetrical ganglion of the right side by means of the crossed 
branch of the commissures, and also by the left ganglion by means of 
the pallio-branchial. It would appear that the gill which is primitively 
innervated by the right ganglion has a tendency to be innervated ex- 
clusively by the left, for the branch from the right side is very delicate, 
and seems to be undergoing atrophy. 
* Arch. Zool. Exper., i. (1893) pp. 221-66 (2 pis.). 
