l.jo HELIX — OLl-ACTORV AXl) Vli^l’AL ORGANS. 
The general sensil)ility has its seat in the whole external integn- 
nient, over which tine setitbrin nenru-ei)ithelial cells are profusely 
distributed, whose irritability renders the surface excessively sensitive 
to tactile inii)ressi(nis and i)robal)ly also to other intluences ; its 
special develojunents are however more particularly localized in the 
anterior region of the body and on its exsertile appendages. 
The Olfactory sense is perhaps the most important faculty possessed 
by the mollusca and is functionally operative and efficient at com- 
paratively great distances ; it is exercised 
by the olfactory organs or rhinophores, 
situate at the distal extremity of the dorsal 
or posterior tentacles, and innervated from 
the cerebral ganglia, the nerve from which 
ex])ands on its course within each tentacle 
to foi’iu a large olfactory ganglion from 
which arise the numerous nerve ramitica- 
tions terminating in the layer of epithelial 
olfactive cellules congregated together on 
the somewhat more elevated epithelium 
spread over the apical surface of the dorsal 
tentacles. It is probable, however, that the 
lower or anterior tentacles, which are well 
and similarly innervated, aid in the exercise of the olfactory sense 
in addition to their tactile function. 
The Eves are two in number and ohli(|uely 
placed at the tijts of the ilorsal tentacles, 
hence called ommatoi>hores or eye-hearers ; 
they are coiistituteil by a retina which arises 
i’roin an invagination of the tegumentary 
ei)ithelium and which contains .sensorial and 
pigmented rods, arranged j)er])endicnlaily 
to the optic axis of the eye ; the oval 
crystalline lens is a cuticular formation 
derived from the retinal epithelium and 
does not entirely till the optic cavity, hut 
is enclosed by a less dense cuticular sub- 
stance, termed the vitreous body or humour. 
The optic nerve springs from the cerebral 
ganglion and reaches the oi)tic bulb in the rear, forming an enlarge- 
opn. 
Fk;. 311. — Eye nf Ucli.x 
poniatiay highly magnified (after 
Simroih). 
r. inner cornea ; cii. cutis : cJ. 
crystalline lens; <’/. epilhelium, 
becoming thin and transparent 
and forming the outer cornea ; 
o.ju. outer membrane or sclera; 
op.n. optic nerve ; ret. retina ; 
t.n. tentacular nerve. 
Fit;. 310. — Rhinophore of 
fleli.v pomatia^ highly magni- 
fied (after Flemming). 
e. eye ; g.c. ganglion cells ; 
g^.s. ganglionic layer ; ol.ep. olfac- 
tory epithelial cellules ; ol.^. 
olfactory ganglion; op.n. optic 
nerve; pigment cells; r. 
eye retractor ; i.r. tentacular 
retractor. 
