THE MORPHOLOGY OF THE MOLLUSC A. 55 



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 Polychsete ancestors (18a), 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 

 been established by the facts and arguments which have 

 been presented in this article : — 



(1) That the pleural ganglia have not been derived by 



segregation from the ventral or pedal cords. 



(2) That the pleural, visceral, and abdominal ganglia 



of Gastropoda form a group of dorsal nerve- 

 centres — the two former owing to their dif- 

 ferentiation in the immediate neighbourhood of 

 the velum, and the latter owing to its differentia- 

 tion from the mid-dorsal wall of the body (floor 

 of mantle-cavity). 



(3) That the dorso-lateral nerve-ring of Amphineura 



is primitive and is represented in other groups 

 of Mollusca by both the pallial and visceral 

 nerve loops, or their derivatives. 



(4) That the sub-intestinal position of the visceral loop 



in all groups except the Amphineura is a 

 secondary one, which has been rendered possible 



