OLFACTOKY ORGANS — RlIINOPHORES. 
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derived from it. In some marine Strei)tonenra the osphradial ridge 
1)}^ specialization acqnires bilateral pectinations and, from its resem- 
blance to the gills, was formerly known as the Parabrancbia, being- 
supposed to represent the atrophied primitively left ctenidium, bnt 
in other groups it may be a simple or bifurcate vibratile pit or 
invagination, with its special ganglion placed at the bifurcation or at 
the blind end. 
The Riitnophores (fn^, fnyos, nose; (f>e:po),to bear) are the olfactory 
organs of the head, and in the Stylommatojdiora are i)laced in the 
more elevated ei)ithelium at the apex of the tentacles, a situation 
which seems to be more es])ecially a characteristic of the terreskial 
snails. An olfactory ganglion is jdaced near the distal end of each 
Fig. 448. — Rhinophore or cephalic olfactory 
orjjan of Cyclostoina clcgans (Miill.) x 30 
(after Garnault). 
cii. cuticle ; olf, c/>. olfactory epithelium ; 
olfactory nerve with terminal hranchlets; 
;//. t. muscular and connective tissue with 
blood sinuses. 
Fig. 440. — Isolated olfactory sensory cells 
from the rhinophore or cephalic olfactory 
organ of Cyclostoma clcgans (Miill.), highly 
magnified (after Garnault). 
tentacle and gives origin to numerous fine nervous ramifications, 
which reach the delicate external epithelium and terminate in 
numerous sui)erficial olfactory cellules. The olfactory ganglia are 
connected to special lobes of the cerebral ganglia, and associated with 
the optic and acoustic nerves which arise from the same lobes. 
The Supra-pedal gland has been, and by some authorities is still, 
regarded as the chief seat of the olfactory sense, owing to its rich 
innervation and the presence of a number of sensory cells in the lumen 
of the gland. 
The Streptoneura, which are essentially a(piatic mollusks, possess 
the osphradium, usually in the form of a filiform, and often pectinate, 
ridge, placed within or near the entrance to the resinratory cavity, 
upon a nervous cord or a small ganglionic enlargement, arising from 
