OLFACTORY ORGANS — GASTROPODA. 
227 
the supra-iiitestinal ganglion, which is the hoinologue of the right 
pallia] or visceral ganglion of the Euthynenres. 
In CijcJostoma and other species, which have comparatively recently 
become adapted to a terrestrial life and aerial respiration, the rhino- 
phores have become developed and acquired functional efficiency and 
an organization quite similar to those of the Stylommatophora or 
typical land snails, although the osphradium associated with their 
primitively aquatic mode of life is also recognizably present, but 
probably undergoing degeneration. 
In some of the more primitive Streptoneura, other sensiferous areas, 
formed by congregations of sensory cells and bearing sensory setin 
are found near the bases of the epipodial tentacles, receiving their 
innervation from the pedal ganglia. 
In the Stylommatoi^hora, although the anterior tentacles possess 
some degree of olfactory power, this sense is more especially concen- 
trated in the dorsal pair, the bulb-like terminations of which are 
especially rich in fine club-shaped sense cells, with rods placed 
amongst specially modified epithelial cells, the somewhat clavate 
olfactoiy ganglion in each rhinophore being placed close to the bulbous 
distal end, and giving rise to several short, thick and divergent 
nervous branches, wdiose numerous ramifications form a beautiful and 
delicate tuft of nervous fibrils, distributed upon a delicate gTanular 
membrane beneath the moist integument at the apex of the tentacle. 
The ganglion vardes in volume in correspondence with the keenness 
or feebleness of the olfactory sense and in some Hijalhi'm is scarcely 
noticeable, while in Helix piscina it has four times the thickness of 
the olfactory nerve. 
The osphradium, being an organ more especially adapted to aquatic 
life, is generally absent in the Stylommatophora ; it is, however, still 
retained in a feebly-developed state at the lower posterior corner of 
the lung cavity in Testacella and by some Helices. It has also been 
detected to be present during development in Limax. 
In the Basommatophora, the tentacles were considered to be the 
seat of this faculty by Lespes and Moquin-Tandon, the nerves are 
however not so decidedly concentrated as in the Stylommatophora, 
but more equally distributed over their whole extent, the superficial 
surface of the tentacles being increased by elongation in Planorbis 
and by attenuation and dilatation in Limnwu, although there is stated 
to be an olfactory ganglion near the tip, quite removed from the eye. 
