46 SCIENCE PROGRESS. 



before their fibres have grown out (which is not a rare 

 embryological phenomenon), there should be nothing in- 

 comprehensible in their fibres taking the shortest route and 

 meeting beneath the gut instead of over it. Clearly, there- 

 fore, the ventral position of the visceral commissure in 

 most Mollusca by no means precludes the possibility of the 

 essential homology between the visceral loop of these 

 forms and part of the pleuro-visceral ring of Amphineura. 



The other differences between the visceral loop of most 

 Mollusca and the pleuro-visceral ring of Amphineura are- 

 principally differences in the degree of segregation and 

 concentration of ganglion-cells and nerve-fibres. The 

 pleuro-visceral ring of Chiton 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, viz., 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 hypothesi 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- 



