14 SCIENCE PROGRESS. 
the attention of its discoverer. In Paludina v. Erlanger 
figures the pleural ganglia arising from the ectoderm on 
each side of the body at a point just outside the velar area, 
but in actual contiguity with the cells of the ciliated ring. 
In Bithynia: (7; Vat. xxvi., fie. 16)ehe fiouresathe ssame 
condition of things for the pair of visceral ganglia. The 
only difference in origin between the two ganglia is that the 
visceral ganglia arise behind the pleural ganglia. If the 
Molluscan veliger possessed a nerve-ring beneath its proto- 
troch (velum), as occurs in the trochosphere of the Annelida, 
it is quite clear that the pleural and visceral ganglia of 
Lithynia and Paludina would represent a series of gangli- 
onic thickenings along the course of the nerve-ring. Apart 
from this inference, however, the topographical relations to 
which I have called attention seem sufficient to establish 
the proposition that the pleural and visceral ganglia, and, 
as I shall show directly, the abdominal ganglion also, of 
Gastropods—and, therefore, of other Mollusca—belong to 
a group of dorso-lateral nerve-centres quite distinct from 
that which is represented by the ventral or pedal cords. 
Here again we are reminded of the direct continuity of the 
pleural and visceral nerve-centres in the Amphineura. 
Development of the abdominal ganglhon.—In Chiton, 
as Kowalevsky has shown (15), the unpaired abdominal 
ganglion, or, as it 1s often called, the visceral ganglion, 
arises by a proliferation of the ectoderm at the hinder pole 
of the embryo, dorsally to the site of the future proctodzeum. 
In the adult this ganglion is simply a special concentration 
of ganglion-cells on the supra-anal portion of the pleuro- 
visceral ring. 
The abdominal ganglion of Gastropods is also situated 
at the hinder end of the visceral loop, but lies of course 
ventral to the gut. Can these two ganglia be regarded as 
homologous ? 
If Molluscs were mere mechanical models the answer 
would be undoubtedly in the negative; but embryology 
