THE MORPHOLOGY OF- THE MOLLUSCA. 19 
Opisthobranchs and Pulmonates have descended from 
Prosobranch ancestors, and the right-sided position of the 
gill-plume in Opisthobranchs is not primitive, but the result 
of a secondary process of detorsion. 
Orthoneurordism.—WV ithout going further into the 
matter it may also here be mentioned that the supra-in- 
testinal commissure has been recently discovered in various 
species of Nerita, Neritina, and Navicella by Boutan (2), 
Bouvier (3a), and Haller (11)—a discovery which de- 
stroys the last refuge of orthoneurism in Prosobranchiate 
Gastropods. Streptoneurism may now be affirmed of all 
Prosobranchiate Gastropods. 
Origin of the Molluscan nervous system.—The attempts 
of previous writers to explain the relations of the nervous 
system of Mollusca have been based almost exclusively 
upon comparisons with the fully constituted nervous 
systems of such types as the Turbellaria and Annelida. 
With Thiele’s theory of the Turbellarian ancestry of the 
Mollusca I have already dealt, and I do not propose to 
deal with the Annelidan hypothesis, since this theory can- 
not provide any satisfactory explanation of the high develop- 
ment of the pleuro-visceral nervous system of the Mollusca. 
Those authors who, like Thiele and Pelseneer, homologise 
both the pleural and pedal centres of the Mollusca with the 
ventral cords of Annelids, base their view upon the sup- 
posed origin of the pleural centres from the pedal cords. 
This derivation I have already shown in this article to be 
completely erroneous. Pelseneer's theory of the origin of 
the Mollusca from Polychete ancestors (18@), and all 
theories which seek the origin of the Mollusca in the 
specialised representatives of any of the vermiform groups, 
may at once in my opinion be dismissed from considera- 
tion. 
Apart from matters of minor importance it will, I think, 
be conceded that the following cardinal points in regard 
to the morphology of the Molluscan nervous system have 
