NERVOUS SYSTEM — PELECYPODA. 
215 
The Basommatophora do not exhibit so marked a concentration of 
the ganglia as is shown by the Stylommatophora, bnt Flanorhh 
displays the same constriction of the cerebro-visceral nerve ring 
Fig. 4*24. — Semi-schematic view of the prosoma of LbnaXy 
showing the arrangement of the ganglia, nerves and other 
organs and their relation to the protrusible and retractile 
buccal bulb, X 3 (after Pelseneer). 
a. abdominal ganglion ; c. cerebral ganglia, with the in- 
fero-posterior buccal ganglia and also showing nerve pro- 
longations to the eyes, rhinophores, labial lobes, Semper’s 
lobes, etc. ; (e. oesophagus ; pedal ganglia ; p.gl. pedal 
gland ; pL pleural ganglia ; S.l. Semper’s lobes ; u. anterior 
aorta : v. visceral or pallial ganglia. 
around the oesophagus as exists in Succinea, 
Fig. 425. — The buccal bulb of 
Succinea pittr.'s(X“)i showing the 
close constriction of the cerebro- 
^'isceral nerve ring around the 
oesophagus, cephalic retractors and 
saliv'ary ducts, owing to the 
shortening of the cerebro-pleural 
connectives, which totally prevents 
the retraction of the buccal bulb 
through the nerve ring, X 8. 
which eciually prevents the 
withdrawal of the buccal bulb. 
In the Pelecypoda the nervous system is c^uite symmetrical and 
apparently simpler in character than in the Gastropods, but this 
chiefly arises from the fusion and combination of ganglia which are 
Diagrammatic dorsally oblique views of the 
nervous system, to show the mode by which the 
apparently simpler nervous organization of the 
Pelecypoda has arisen from the more complicated 
arrangement of their assumed ancestor. 
Fig. 426. — Nervous system of a primitive 
mollusk, showing the posteriorly directed buccal 
ganglia, the distinct pleural and pallial centres, 
and the paired abdominal ganglia. 
Fig. 427. — An intermediate stage, showing the 
approximation of the pallial and abdominal and of 
the cerebral and pleural ganglia, which eventually 
fuse and form the visceral or parieto-splanchnic 
and the cerebro-pleural ganglia respectively of the 
Pelecypoda, the cerebro-pedal and the pleuro- 
pedal connectives also combining in a part of their 
course owing to the increasing approximation of 
the pleural to the cerebral ganglia ; the buccal 
ganglia also tend to combine with the cerebro- 
abdominal nerve cords. 
Fig. 428. — Nervous system of a Pelecj’pod, 
showing the completion of the fusion of the cere- 
bral and pleural ganglia, and of the pallial and 
abdominal ganglia to form the cerebro-pleural and 
visceral ganglia respectively. The buccal ganglia 
and connectives are completely fused with the 
commissures from the cerebro-pleural ganglia, and 
the cerebro-pedal and cerebro-pleural connectives 
have also become intimately joined together. 
ai.g^. abdominal ganglia ; b.g". buccal ganglia ; c.c. cerebral commissure ; c.g. cerebral ganglia ; 
c.p.c. cerebro-pedal connective ; c.pLp.c. cerebro-pleuro-pedal connective ; c.pl.u.c. cerebro-pleuro- 
visceral commissures ; c.pl.c. cerebro-pleural connective ; c,pl.g. cerebro-pleural ganglia ; pa.a.c. 
pallio-abdominal commissures ; Pa.g. pallial ganglia ; p.g, pedal ganglia ; pLg. pleural ganglia ; 
pLpa.c. pleuro-pallial commissures ; pl.p.c. pleuro-pedal connective ; v.g. visceral ganglia. 
often distinctly separated in Gastropods. Usually there are only 
three distinct but widely separated nervous masses, known as the 
