18 MANUAL OF THE MOLLTTSCA. 



tritonia and eolis, 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. Yalenciennes 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, 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 Encephala the tongue is armed with spines, 

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

 very 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- 

 Fig. 12. Lepton squamosum^ j • i n jt •^ • i / 1 



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 reojards the membranous lamellce between the oral tentacles and in 

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



t Fig. 12. Lepton squamosum, Mont., from a drawing by JMr. Alder, in the Srit^ 

 Mollusca ; copied by permission of Mr. Van Voorst. 



