48 SCIENCE PROGRESS. 



view by which the pleural ganglion is regarded as a 

 derivative of the pedal cords (Bouvier, Pelseneer, etc., 

 passim). This view is founded on the fact that in the lower 

 Gastropoda (Docoglossa and Rhipidoglossa) the pleural 

 ganglia are directly continuous with the anterior ends of 

 the pedal cords, while in the higher types the pleural ganglia 

 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 Lottia show— or rather in the reo-ion 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 Lottia with that of Chiton (Thiele, 

 2 3 ? 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 

 Zoologie and the Bulletin Scientifique de la France et de la 

 Belgique. 



