MANUAL OP THE MOLLTJSCA. 



which M. Valenciennes regards as the organ of smell*. Messrs 

 Hancock and Erableton attribute the same function to the la- 

 mellatecl tentacles of the nudibranchs, and compare them withi 

 the olfactory organs of fishes. 



The labial tentacles of the bivalves are considered to be or- 

 gans for discriminating food, but in what way is unknown (fig. 

 18. I. .) % 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 animal- 

 cules, and even the sharp spicula of sponges. In some instances, 

 however, the oral orifice is well guarded, as in pecten (fig. 10.) 

 In the Enceplmla, the tongue is armed with spines, employed 

 in the comminution of the food, and cannot possess a very de- 

 licate sense. The more ordinary 

 and diffused sense of touch is pos- 

 sessed by all the mollusca ; it is 

 exercised by the skin, which is 

 everywhere soft and lubricous, 



and in a higher degree by the 



/.. P-T i i AC -i \ Fig- 12. Lenton Squamosum.-\ 



fringes of the bivalves (fig. 12), 



and by the filaments and tentacles (vibraculd) of the gasteropods ; 

 the eye-pedicels of the snail are evidently endowed with great 

 sensitiveness in this respect. That shell-fish are not very sensi- 

 ble 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 moUusca are prin- 

 cipally connected with the skin, which is exceedingly contractile 

 in every part. The snail affords a remarkable, though familiar 



* Mr. Owen regards the membraneous lamella between the oral tentacles 

 and in front of the mouth, as the seat of the olfactory sense. See Fig. 44. 



f Fig. 12. Lepton sqaumosum Mont., from a drawing by Mr. Alder, ia 

 the British Mollusca ; copied by permission of Mr. Van Voorst. 



