10 SCIENCE PROGRESS. 
pleuro-visceral ring of Chzton represents a very primitive 
nervous system, characterised by the more or less even 
diffusion of ganglion-cells over the whole length of the 
cord, while the nerves arising from it are not united into 
large trunks, but are given off at repeated intervals in a 
manner which is almost metameric. The nerves springing 
from it innervate the same parts of the body as the com- 
bined pleural and visceral ganglia of Gastropods and other 
Molluscs, vzz., mantle, ctenidia, intestine, heart, nephridia, 
and gonads. But if, after the reduction of the ctenidia to 
a single pair, we imagine a process of segregation to set in 
between these various elements, the more strictly visceral 
centres would become separated from the superficial pallial 
centres, and would assume a deeper position in the body. 
The law of concentration would apply in this as in other 
cases of evolution of nervous systems (3), and the result of 
the whole process would be the differentiation of a visceral 
nervous system, consisting of ganglia and commissural 
fibres, out of the primitively mixed and diffuse pleuro-visceral 
system. If the primitive relations to the gut and ring-like 
form were retained at all, they would be retained, not 
necessarily by the visceral system, which has ex hypothesz un- 
dergone considerable changes, but by the pallial (= pleural) 
system, which has undergone no change, except possibly 
one of incipient concentration. 
The position of the commissural fibres of the visceral 
ganglion in relation to the gut becomes a matter of sub- 
ordinate importance if the evolution of the nervous system 
has proceeded upon these lines, as will be made evident 
later on. As a matter of fact the visceral commissure is 
situated below the gut—a relation which is possibly fore- 
shadowed in Chztox by a connection beneath the gut of the 
two gastric nerves described by Haller (8). 
Pelseneer (19) indeed goes so far as to identify these 
gastric nerves of Cztom with the visceral commissure of 
Gastropoda and Pelecypoda; but the considerations which I 
