18 MANUAL OF THE MOLLUSCA. 
tritonia and colis, none of the mollusca have been observed to 
emit sounds. (Grant.) 
Sense of Smell. This faculty is evidently possessed by the 
cuttle-fishes and gasteropods; snails discriminate their food by 
it, slugs are attracted by offensive odours, and many of the 
marine zoophaga may be taken with animal baits. In the pearly 
nautilus there is a hollow plicated process beneath each eye, 
which M. Valenciennes regards as the organ of smell.* Messrs. 
Hancock and Embleton attribute the same function to the 
lamellated tentacles of the nudibranchs, and compare them with 
the olfactory organs of fishes. 
The labial tentacles of the bivalves are considered to be 
organs for discriminating food, but in what way is unknown 
(Fig. 18, 7, t). The sense of taste is also indicated rather by the 
habits of the animals and their choice of food than by the 
structure of a special organ. The acephala appear to exercise 
little discrimination in selecting food, and swallow anything 
that is small enough to enter their mouths, including lying 
animalcules, and eyen the sharp spicula of sponges. In some 
instances, however, the oral orifice is well guarded, as in pecten 
(Fig. 10). In the Encephala the tongue is armed with spines, 
employed in the comminution of the food, and cannot possess a 
yery delicate sense. The more ordinary and diffused sense of 
: touch is possessed by all the 
mollusca ; it is exercised by the 
skin, which is everywhere soft 
and lubricous, and in a higher 
degree by the fringes of the bi- 
valves (Fig. 12), and by the fila- 
ments and tentacles (vibracula) 
of the gasteropods; the eye- 
pedicels of the snail are evidently 
endowed with great sensitiyeness in this respect. That shell-fish 
are not very sensible of pain, we may well believe, on account 
of their tenacity of life, and the extent to which they have the 
power of reproducing lost parts. 
Muscular System. The muscles of the mollusca are principally 
connected with the skin, which is exceedingly contractile in 
eyery part. The snail affords a remarkable, though familiar 
instance, when it draws in its eye-stalks by a process like the 
Fig. 12. Lepton squamosum.f{ 
* Mr. Owen regards the membranous lamelle between the oral tentacles and in 
front of the mouth, as the seat of the olfactory sense. Sce Fig. 51 
{ Fig. 12. Lepton squamosum, Mont., from a drawing by Mr. Alder, in the Brith 
Mollusca ; copied by permission of Mr. Van Voorst, 
