THE MORPHOLOGY OF THE MOLLUSCA. 17 
Mr. Kerr has undoubtedly failed to appreciate the true 
nature of the posterior sub-cesophageal loop of Maztzlus, as 
well as the relation of the visceral nerves to the pleuro- 
visceral cords of Chzton. The explanation of the Cephalo- 
pod nervous system is most readily found by comparing it 
with that of Dextalium, whose organisation in many respects 
supplies connecting links between that of the Cephalopoda 
and that of the primitive pra-torsional Gastropod or 
primitive Pelecypod. In Dentalium (22, p. 401) we find 
a pair of post-anal prolongations of the visceral nerves 
precisely resembling those described by Kerr in Nautilus ; 
yet in Denéalium, owing to the smaller degree of concen- 
tration or cephalisation which has taken place in the 
nervous system, it is easy to see that the typical sub-intes- 
tinal visceral commissure exists as in Gastropods and 
Pelecypods. The posterior sub-cesophageal nerve-mass 
of Cephalopods has clearly been produced, not, as Mr. 
Kerr suggests, by a secondary fusion of the pleuro-visceral 
nerve-masses of the two opposite sides, but by a simple 
shortening of the visceral loop as it occurs in Dentalium. 
This would bring the visceral ganglia into continuity 
with the pleural ganglia and with one another,—a process 
of condensation with which we are already familiar in the 
Tenioglossa and the Euthyneura among Gastropoda. 
It may here be mentioned that Willey’s simultaneous 
account (26) of the visceral nerves of Nautilus, while con- 
firming Mr. Kerr’s observations as to the existence of post- 
anal prolongations of a pair of visceral nerves, differs from 
his statement as to their origin. Willey states that the 
nerves supplying the post-anal papilla arise independently 
from the sub-cesophageal visceral loop, although at their 
origin they are adjacent to the branchial nerves and for a 
large part of their course are actually contiguous with them. 
The significance of this separation is not remarked upon by 
Willey ; but if the separation really exists it is certainly a 
difficulty in the way of his contention that the post- 
