THE MORPHOLOGY OF THE MOLLUSC A. 47 



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 Chiton 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 Chiton with the visceral commissure of 

 Gastropoda and Pelecypoda; but the considerations which I 

 have emphasised above show that the typical visceral nerves 

 and commissure have not yet arisen in the Amphineura ; 

 they do not arise, in fact, until the branchial, nephridial, 

 genital and enteric branches of the primitive pallio-visceral 

 cords are all united into one common trunk. There is 

 some doubt, moreover, as to the existence of the gastric 

 nerves described by Haller, since two investigators, Plate 

 (20) and Thiele, have been unable to discover them in 

 species of Chiton examined by themselves. 



A valuable contribution to this part of the subject is 

 contained in Haller's recent Studien (11). In the 

 common cyclobranchiate types of Limpet the pallial nerves 

 are separate from one another behind, and seem to be 

 mere outgrowths of the pleural ganglia (Bouvier, 3, p. 19); 

 but in Lottia, one of the more primitive monobranchiate 

 forms, Haller shows that the pallial nerves of the two sides 

 are directly continuous with one another posteriorly, and make 

 a complete arch round the edge of the mantle. They are 

 moreover not mere nerves, since they consist of a core of 

 fibres surrounded by an outer coating — discontinuous, it is 

 true — of ganglion-cells. They are clearly the posterior 

 continuations of the pleural ganglia, and represent the re- 

 mainder of the pallio-visceral nerve-ring of the Amphineura 

 after the separation of the visceral elements. This view is 

 further borne out by the existence of several connectives 

 between the pallial ring and the pedal cords in addition to 

 the stout ganglionic connective which in higher forms 

 becomes the persistent pleuro-pedal connective. 



The pleural ganglion. — Haller's discovery recorded in 

 the preceding paragraph shows clearly the error of the 



