OLFACTORY ORGANS — OSPHRADIUM. 
225 
The originally paired cdiaracter of the ospliradiuiu is still retained 
by the Pelecypods and the more archaic Streptonenra, bnt in the 
Gastropoda of onr fauna the torsion undergone by the body has, 
in dextral species and individuals, 
resulted in the atrophy and loss of 
the osphradinin and other organs of 
the primitively left side and of those 
of the primitively right side in such 
forms as are sinistrally coiled. 
The osphradium seems to be more 
particularly correlated with an 
acpiatic habit of life and the i)re- 
sence of a luanchial cavity, as it is 
j)ractically absent in terrestrial 
genera, and in those aquatic forms 
deficientof a pallia! chamber, though 
still persisting in some species 
that have comparatively recently 
relimprished an aquatic life and 
branchial respiration, but when thus present it always corresi)onds 
more or less closely in position with that of the vanished ctenidium, 
with which it was primitively associated. In its simplest form the 
Fig. 441. — Osphradium or palHal olfactory 
organ of Planorbis cornens (L.), highly 
magnified (after Felix Bernard). 
bi.s. blood sinus ; c. olfactory pit ; c.t. con- 
nectiv'e tissue : ^.c. olfactory' ganglion cells; 
s.cp. olfactive epithelium. 
Figs. 442. 443. 414. 445. 446. 147. 
Various forms of cells from the osphradium or olfactory pit of Liuincra stag'nai/s {h.). highly 
magnified (after Simroth). 
Fig. 442. — Goblet cell. Fig. 443. — Ciliated cell. Fig. 444. — Group of cells, one united to a nerve 
cell by a nerve thread. Fig. 445.~Group of cells with yellow pigmented nuclei and ner\e pro- 
longations. Fig. 446. — Olfactory bipolar and Fig. 447. multipolar ganglion cells. 
osphradium is merely a., localization of suitable epithelial sensory 
cells, connected with the integument and placed within or near the 
entrance to the respiratoiy cavity, upon the course of the branchial 
nerve, or upon a .special o.sphradial nerve or ganglionic enlargement 
2/1,97. 
p 
