•214 
NERVOUS SYSTEM — STllEPTONEURA AND EUTHYNEURA. 
In the Euthyneura the male organ is innervated froiiit he cerebral 
and not from the pedal ganglia, and the pleuro-abdominal commis- 
sures do not exhibit the crossing that so markedly characterizes the 
Streptonenres, bnt Prof. Pelseneer lias demonstrated the probability 
of the former existence of a similar torsion which is shown not only by 
the retention of a partial streptoneury by some of the more arcliaic 
})nlmonate genera, bnt by the right pallia! ganglion still occupying a 
more elevated iiositiou than the left, perhaps due to its previous 
su})ra-intestinal position. 
Fig. 423. — Diagram of the nervous system of an Euthyneure, Lijnntea pcrcgra (Milll.), showing 
the concentration of the visceral ganglia and the dextral position of the morphologically right 
osphradium (modified after Spengel). 
abdominal ganglion ; buccal commissure; h.g. buccal ganglia; cJkc* cerebro-buccal 
connectives ; c,g, cerebral ganglia ; c.p.c, cerebro-pedal connectives ; l.d. left dialyneurous nerve ; 
t?/. otocyst ; osphradium ; p.g. pedal ganglia ; pci^g- pallial ganglia ; pleural ganglia ; 
pl.p.c. pleuro-pedal connectives; p/.pa.c. pleuro-pallial commissures \p.a.c. pleuro-abdominal com- 
missures ; r.d. right dialyneurous nerve ; sb.g» subintestinal ganglion ; sp.g. supra-intestinal ganglion. 
Ill the Stylommatophora the greatest aniount of specialization 
is exhibited, the various ganglia having become more or less 
closely aggregated around the jdiarynx, behind the buccal bulb, by 
the shortening of the conneetives and commissures, the buccal bulb 
being capable of withdrawal or protrusion through the cerebro- 
visceral nerve loop, as in Ilelix asj^ersa and other species, but in some 
genera, as Sacciuea, the cerebro-iileiiral connectives have by specializa- 
tion become so short and the cerebro-visceral opening so contracted 
thereliy that the buccal bulb cannot be withdrawn through the nerve- 
ring, and is therefore practically immoveable. 
