■238 
AUDITORY OR EQUILIBRATING ORGANS — EUNCTION. 
seiisoiT hairs u])on the walls of the sac indiiciHg or leading to the 
suitable reflex actions of the animal. 
Although termed auditory organs, they are prohabl)'’ only sensitive 
in a very limited degree to sound audible to human ears, but have a 
Fig. — Porlion of ilie otocyst 
of Sphariufu comeimt (I,.), highly 
magnified (after Siniroth), showing 
the otolith, o/., in contact with the 
sensory hairs. 
Fig. 4G7. — Terminal nerve cells 
from the walls of the otocyst of 
Cyciostofiia i'h'gnns (Mull.), highly 
magnified (after Garnault). 
much keener perception of any disturbance of the inhabited medium 
or of the surface to which the foot may be applied, d'heir chief function 
is undoubtedly to regulate the orientation and preserve the equilibrium 
of the body by aiding in the percei)tion of the horizontal position 
during locomotion, probably by reflex muscular action, induced by 
the varying movements of the otoliths upon the sensory nerve-endings. 
'I'his function is the more likely to be the predominant one, as this 
organ becomes atrophied in the adult stage of the flxed forms, such as 
Di ■eissensid, though present in the free swimming early stages of growth. 
The otocysts are most highly developed in the active forms, and in 
the reptant .siieciesare usually found near to or actually in contact with 
Fig. 408. — Diagram of the 
nervous system of IVjtcu/a^ 
showing the open canal of 
invagination from the otocysts 
to the exterior, and incidentally 
the still separate cerebral and 
pleural ganglia and their con- 
nectives (after Pelseneer). 
ot. otocyst; ot.d. otocystic 
duct ; ab.g^. combined pallial 
and abdominal ganglia ; c.g. 
cerebral ganglia ; c./ c. cerebro- 
pedal connectives ; 
cerebro - pleuro • pedal connec- 
tive ; osp. osphradium ; 
pedal ganglia ; pleural 
ganglia; pl.p.c. pleuro- pedal 
connective ; pl.v.c. pleuro- 
visceral commissures. 
the pedal or locomotory 
ganglia, Imt in the natant 
forms they tend to approach 
the cerebral ganglia, fiom 
which centre they receive 
their innervation. In our 
Oastropods they vary in the 
position they occupy, being 
com])aratively remote from 
the pedal ganglia in Cydo- 
stomd ; actually placed upon 
or imbedded within their 
posterior outer surface, as in the Stylommatophora ; or, as in Nerltina, 
may be close to, yet quite distinct from, the ganglionic mass. 
