THE MORPHOLOGY OF THE MOLLUSCA. II 
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 Chzton 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 Loé¢éza, 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 
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 
