• 2-28 OLFACTORY ORGANS — BASOMMATOPHORA AND PELECYPODA. 
with which the oltactory sense is so closely associated in the Helices, 
etc. Lacaze-Duthiers considers the external Imsal enlargement of the 
tentacles to also i)Ossess olfactory power. 
In the L'nnna'idu’ the osphradinm has persisted after the loss of 
the gill with which it was primitively associated, and now exists in 
the form of a ronnded ei)ithelial ridge, placed npon or as a vihratile 
])it, invaginated within a small olfactory ganglion, this olfactory sinus 
lieing simple in the sinistra! genera P/anorbis, etc., hnt 
hifnrcate in the dextral genns LImita’ii, and, as might he expected, 
P/iinorhIs has retained the osphradinm of the originally left side, and 
the innervation hythe left visceral 
ganglion, while L/iniufti has re- 
tained that of the original right 
si<le and receives its nerve supply 
from the right visceral ganglion 
Init being an aipiatic sensory organ, 
whose role is to guard the entrance 
to the pallial cavity, it has (piitted 
its ])rimitive position inside the 
mantle chamber and l)ecome ex- 
terior to it, partly due to the 
contraction of the iiallial opening in the ancestors of the Pnlmonates, 
hnt mainly because water does not now normally enter the pallial 
cavity of the Lhantriilv. In bdpli(ni(iri(i,i\ marine Ba.sommatophore, 
in which the water now enters the lung, the o.s])hradinm is i)artially 
within the cavity. 
iSimroth at one time considered the os])hradinm to he the medinm 
by whicdi these pnlmonate apnatic snails became atapiainted with 
their arrival at the surface of the water for the pnri)Ose of respiration. 
In the I’elecypoda, S})engel has demonstrated in Anodontd and 
other siiecies a i)aired osjdiradial structure richly supiilied with nerves, 
situate upon the roof of the inhalent siphon, near the origin of the 
branchial nerves, and close to the posterior addnctor, analogous 
in position and structure with the osidiradia of the axpiatic ({astroijods. 
It is a specially moditied crest of epithelium, consisting of a thin layer 
of elongated sensory cells, })laeed near to and innervated from the 
visceral ganglia, hnt according to Pelseneer tlie nervous connection is 
really with the cerehro-pleural ganglia, by means of the cerehi’o- 
plenro-visceral commissures. 
Fig. 150. — Litumca percg^'a (Mull ) ex- 
tracted froni its shell and the mantle flap turned 
back to show the position of the osphradinm 
in relation to the pulmonary orifice, etc., X 3. 
/. foot : f.o. female aperuire ; w, mantle 
flap : m.o. male aperture ; os. osphradinm ; 
pulmonary orifice ; sp. spire. 
