NERVOUS SYSTEM — DIALYNEURY. 
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constituents of tliis centre have by specialization become quite 
separateil and fused with the cerebral ganglia, are probably formed 
by the fusion of the pallial and abdominal centres of the primitive 
mollnsk, the moities of this centre being however still separate in the 
])rimitive genus Xaculii. The visceral ganglia are usually in contact 
with and ventral to the posterior adductor muscle and innervate the 
gills, the posterior adductor, and the viscera generally. They also 
give origin to the posterior pallial plexus, which innervates the 
respiratoiy oribces and the mantle margins and fuses midway therein 
with the pallial nerve from the cerebro-pleural ganglia. 
'fbe term (Irtboneuroid was formerly applied to the nervous system 
of Xeritiud and Ilvlkhui, the supra - inte.stinal commissure and 
ganglion being stated to be debcient in those groups, and the 
crossing of the pleuro-abdominal commissures, characteristic of the 
Strei>tonenra, being consecpiently denied to exist. This belief is, 
however, erroneous, as no truly ortboneuroid mollnsks are known. 
Dialyneury exists only in the Streptoneura and consists in the 
establishment of a direct nerve connection between the constituent 
ganglia of the visceral centre, morphologically belonging to 0})posite 
sides of the body ; this connection arising on the left side by the 
Fig. 438. — Nervous system of Cyclostoma 
elcgans (Mull.), illustrating Dialyneury (after 
Lacaze-Duthiers). 
Ld. nerve from the left pleural ganglion fused 
with the pallial branchial nerve, arising from the 
supra-intestinal ganglion and forming left Dialy- 
neury ; r.d. nerve from the right pleuro-pedal 
connective anastomosing with the pallial nerve 
from the subintestinal ganglion and constituting 
right Dialyneury. 
ah.g. abdominal ganglion ; b.c. buccal commis- 
sure ; h.g. buccal ganglia ; c.b.c. cerebro-buccal 
connective; cerebral ganglia; c.p.c. cerebro- 
pedal connective ; ot. otocyst ; osp, osphradium ; 
p.g. pedal ganglia;//.^, pleural ganglia ; //./.r. 
pleuro-pedal connective ; p/.pa.c. pleuro-pallial 
commissure ; />.a.c. pallio-abdominal commissures ; 
sb.g. subintestinal ganglion ; sp.g. supra-intestinal 
ganglion. 
mantle nerve from tlie left pleural ganglion fusing with the branchial 
nerve emanating from the supra-intestinal or primitively right pallial 
ganglion, thus forming a nerve loop above the alimentary canal ; 
and by a similar junction having been effected beneath it on the right 
side of the body between the pallial nerve arising from the right 
pleuro-i)edal connective, but whose bbres probably originate in the right 
