20 SCIENCE PROGRESS. 
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 _ 
only by the decentralisation of the primitive 
pleuro-visceral nervous system, and its separa- 
tion into special ganglia and nerves, the latter 
being formed ontogenetically as fibrous out- 
growths from the ganglionic centres. 
Venturing now, in conclusion, upon more speculative 
ground, I believe that the embryonic relations, to which I 
have drawn attention, between the pleural and visceral 
ganglia and the ciliated band are of phylogenetic importance. 
It has long puzzled me that the larval forms(trochospheres) of 
two groups so closely allied as the Annelida and Mollusca, 
while presenting a close similarity in general structure, 
should differ so remarkably in regard to their nervous 
system. The Annelid trochosphere has a_nerve-ring 
beneath its ciliated band, while the Molluscan trocho- 
sphere has none. In this respect the Molluscan trocho- 
sphere appears to be less primitive than that of the 
Annelida. The explanation of this now appears to me to 
