24 / 
continuity of nervous tissue throughout more or less extensive regi- 
ons of the body and becoming ever more apparent amongst the 
less differentiated groups. And now the question arises: is it not 
very significant that the lower we descend in the Molluscan sub- 
division the more a system of transverse commissures between the 
longitudinal conductive stems fixes our attention ? 
In Haliotis and Fissurella the pedal nerves are united by trans- 
verse commissures, in Chiton this connection is far more complete 
and elaborate. In Neomenia these transverse commissures were 
similarly found and now in Proneomenia we find in addition to 
the latter a very complete system of transverse commissures on 
both sides connecting the lateral with the pedal nerves. 
Is the concelusion too far fetched that here we have a valuable 
indication that in the primitive Mollusks the nervous system also 
tended towards the plexus-like arrangement, now met with in the 
lower worms and that by gradual centralisation the primary con- 
ductive stems have finally come to preponderate, whereas the com- 
missures disappeared? At the same time the innervation of differ- 
ent .organs and tissues, which in Proneomenia devolves upon 
numerous branches springing from the commissures is at last solely 
brought about by direet nerve-branches from either of the longi- 
tudinal stems. 
The separate origin in the cerebral ganglion of the sublingual, 
pedal and lateral nerves and the absence of any anterior ganglio- 
nic coalescence between them, indicates another point of difference 
between Proneomenia and Neomenia and points to affinities lower 
in the scale. It appears to me that a further confirmation of the 
views here developed is furnished by the histology of the nervous 
system of Proneomenia. Here as in Plathelminths the primary 
nerves are accompanied by a layer of nerve-cells (fig. 43), all along 
the whole of their length. Hansen (7) notices ihe same for Chaeto- 
derma. Even in the commissures nerve-cells are not wholly absent 
(fig. 44), whereas the fibrous nucleus of the principal stems and of 
the commissures eminently resembles the spongeous, loosely interlaced 
complex of nerve-fibres in the corresponding stems of the lower worms. 
