12 SCIENCE PROGRESS. 
gradually move further and further away from the pedal 
ganglia, and, travelling along the cerebro-pleural connectives, 
eventually come into contiguity with the cerebral ganglia 
(Tenioglossa) or even fuse with them to form a single 
cerebro-pleural ganglion on each side (Pelecypoda). 
The close connection between the pleural and_ pedal 
ganglia in the lower forms may now be _ interpreted 
in a different manner. The ganglion-cells which were 
primitively distributed over the whole extent of the pallial 
nerve-ring have been concentrated at the anterior ex- 
tremities of its lateral portions, as Haller’s observations 
on Lottza show—or rather in the region of the first 
pleuro-pedal connective, for the most anterior portion of 
the primitive pallial cords is represented by the cerebro- 
pleural connective. The shortness of the pleuro- 
pedal connecting piece and the great concentration of 
ganglion-cells which takes place at its two extremities 
prevent any sharp demarcation between the pleural and 
pedal ganglia in these lower forms; but a comparison of 
the nervous system of Lot¢za with that of Chztox (Thiele, 
23, p. 387) leaves no room for doubt as to the correct- 
ness of this interpretation, which throws a flood of light 
upon numerous other points which have been difficult to 
understand upon the older views. It explains, for example, 
why the cerebro-pleural and cerebro-pedal connectives 
should be already distinct from each other in the lower 
Gastropods at a stage when the pleural ganglia are in 
actual continuity with the pedal cords, and it sets at rest 
the controversy as to the meaning of the lateral furrow 
in the pedal cords of Rhipidoglossa which has been waged 
with so much skill in the rival pages of the Archives de 
Zoologre and the Bulletin Scientifique de la France et de la 
Leloique. 
Development of the pleural ganglion.—That the pleural 
ganglion is essentially distinct from the pedal is, I think, 
sufficiently clear from the facts of development. Although 
