18 MANUAL OF THE MOLLTJSCA. 



tritonia and eolis, none of tlie moUusca 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 comj^are 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, I, 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 living 

 animalcules, and even the sharp spicula of sponges. In some 

 instances, however, the oral orifice is well guarded, as in pecten 

 (Fig. 10). In the JEncephala the tongue is armed with spines, 

 employed in the comminution of the food, and cannot possess a 

 very delicate sense. The more ordinary and diff'used 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 {vihracula) 

 of the gasteropods ; the eye- 

 pedicels of the snail are evidently 

 endowed with great sensitiveness 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 

 every part. The snail affords a remarkable, though familiar 

 instance, when it draws in its eye-stalks by a process like the 



* Mr. Owen regards the membranous lamella between the oral tentacles and in 

 front of the mouth, as the seat of the olfactory sense. See Fig. 51. 



t Fig. 12. Lepton squnmosum, Mont., from a drawing by Mr. Alder, in the BritJsh 

 Mollusca ; copied by permission of Mr. Van Voorst. 



Fig. 12. Lepton squamosum.] 



