NERVOUS SYSTEM — ZYGONEURY AND SENSORY ORGANS. 
223 
pleural ganglion and the pallial nerve from the primitively left pallial 
or sub-intestinal ganglion, the crossing of the pleuro-abdominal com- 
missures having brought these ganglia into close proximity, although 
prior to it occupying opposite sides of the body. 
Zygoneury is an exaggerated development of Dialyneury, in which 
a much closer api)roximation or even a fusion takes place on one side 
of the body only, of the pleural and pallial ganglia, morphologically 
belonging to opposite sides of the median line, owing to the shortening 
of the pleuro-pallial commissure aud of the anastomosed nerves, whose 
connection constituted Dialyneury ; usually it is the primitively 
Fig. •139. — The nervous system of 
Bucchium undatujn (L.), illustrating 
Zygoneury to the right, and incidentally 
Dialyneury to the left (after Pelseneer). 
sb,g. subintestinal ganglion fused to the 
right pleural ganglion and constituting 
Zygoneury to the right ; supra- 
intestinal ganglion ; l.d. nerve joining 
the pallial nerve from the left pleural 
ganglion to the branchial nerve emanating 
from the supra-intestinal ganglion and 
constituting Dialyneury to the left. 
ab.s^. abdominal ganglion branchial 
nerves ; c.g. cerebral ganglia ; p.g. pedal 
ganglia ; p.n. pallial nerve; pLg. pleural 
ganglia; pLp.c. pleuro-pedal connective ; 
pa.a.c, pallio - abdominal commissures. 
left pallial or sub-intestinal ganglion which becomes closely associated 
with or actually fused to the right pleural ganglion, and constitutes 
Zygoneury to the right ; more rarely the primitively right pallial or 
supra-intestinal ganglion is similarly connected to the left pleural 
ganglion, and is distinguished as Zygoneury to the left. 
THE sensory organs. 
Interspersed among the glandular, ciliated, and simple epithelial 
cells, which cover the external surface of the mollusk, are a number 
of elongate or superficially expanded neuro-epithelial cells, known 
also as Flemming’s cells, which often bear one or more sensory hairs 
at the free end, and vary in number in the various areas, but may be 
congTegated together in certain definite parts of the body, each 
cell being continued at the base into a nerve fibre, which is 
connected with the central nervous system. These cells have not 
