224 
OLFACTORY ORGANS — OSPHRADIUM. 
primitively a definite and precise function, and may respond to 
difi’erent stimuli, but when densely aggregated together in restricted 
areas of the body they form the sensory 
organs which, by the development of 
specialized parts adapted to collect and 
transmit particular forms of stimulation, 
convey to the mollusk more or less 
definite impressions of the character of 
external objects ; thus, the optic nerves 
are only sensitive to rays of light, and 
the olfactory nerve to the i>resence of 
odours, whereas the less specialized sensi- 
bility to tactile impressions is distri- 
buted over the entire surface of the body, which is extremely sensitive 
to the slightest contact. 
Fig. i40. — Sensory and epithelial 
cells, showing their arrangement, 
highly magnified (after Garnault). 
ep,c. epithelial cells ; s.c, neuro- 
epithelial or sensory cells. 
The Olfactive faculty is undoubtedly possessed by the mollusca, 
and almost every part of the animal has been suggested, at one time 
or another, as the seat of this function, l)ut modern investigation 
favours the view that although the tentacles and the sensory area near 
the resi)iratory orifice are the chief seats of this sense, yet the oral 
cavity in llellr, L'nnncca, and some other genera has also some olfac- 
tory power, while the whole soft skin of the animal, which somewhat 
resembles a pituitary memlwane, is probably also more or less sensible 
to the perception of odours. 
In the different genera this sense is more especially concentrated 
in, and exercised by morphologically distinct regions of the body, 
which according to their position are termed Osphradia or Rhinophores, 
the former being more esi)ecially adapted to a(piatic and the latter 
to aerial respiration : where both are present, their different degree 
of functional development is probably in inverse proportion to each 
other, one being probably in process of develo})ment and the other 
undergoing degeneration. 
The OsPHRADiUM (ocrc^paSior, from ocrc^pafro/rat, to smell) Or pallial 
olfactory organ, known as the organ of Lacaze and also as Spengel’s 
organ, is a primitively paired sensory structure, formed by tracts of 
suitably modified ciliated epithelium, usually innervated from the 
visceral or pallial ganglia and placed at the outer base of each ctenidium, 
functioning as an examiner and co-ordinator of the sensations received 
from the stream of inhalent w'ater which bathes the gills. 
