244 
TACTILE ORGANS. 
Tlie Tactile .sense is well developed in the inollnsca, being resident 
in all parts of the external skin, which is very soft and moist and 
acutely sensitive to the slightest contact. This faculty is e.specially 
exercised by externally expanded 
nenro-epithelial cells bearing tufts of 
sensoiy hairs and by long and fusiform 
nucleated cells, which may he pro- 
longed externally into one or more 
hue setiform sensitive processes, and 
internally are directly continuous with 
the nerve tihrils, these cells being 
most strongly and numerously 
developed on the most prominent or 
exposed jiarts of the body naturally functioning as tactile organs. 
In the (histro])oda this sense, though diffused generally over the 
surface of the body, is most actively exercised by the anterior tentacles 
and labial lobes, which are jiossessed of the most delicate susceptibility 
to contact with external objects. 
F'ig. 401. — Section through the anterior 
tentacle of flelix pontaiia L., to show 
the peripheral tactile cells and general 
structure, highly inagnilied (after Flem- 
ming). 
s.c. sensory cells ; cp. epithelium ; /.r. 
pigment ; n.c. large nucleated cells ; 
c. transverse section through muscles. 
Fig. 402. 
Fig. 103. 
Fig. 402. — AtTerent nerve and connected afferent ganglion cell, showing its tactile terminations 
within the external epithelium, X GOO (modified after I’uas). 
Fig. 403. — Terminal nerve cells and Fig. 401 Epithelial ciliated cells from the tentacle of 
/ 'h'ipiira 7'ivipaya (L.), highly magnified (after Simroth). 
In the l’elecy])oda tlie tactile sense cells are distributed over the 
surface of tlie body and niiintle. The tip of the foot seems e.specially 
f 
Fig. 495. — Tentacles of 
the branchial siphon of 
,-i uoiionfa cx^nt'a {\j.\ X 10 
(after Moquin-Tandon). 
sensitive and is probably the most effective 
organ of touch. 'I'lie papillary processes of the 
sijihons and the mantle margins generally are 
exquisitely sensitive to tactile impressions, as 
are the labial tentacles, but the latter are 
perhaps scarcely organs of touch in the sense intended here, 
