MOLLUSCS. 767 



The organs of sense are more developed, as a rule, in this than 

 in the two preceding classes. The head is usually furnished (in 

 most Gasteropoda) on the upper surface with two feelers, on the 

 outer margin of which the eyes, sometimes on two little tubercles 

 or pedicles, are situated. When the eye- supporting pedicles are 

 developed independently, two pairs of feelers are present. In 

 some these feelers are hollow and can be everted like the finger of 

 a glove after they have been drawn in, as may be seen in the com- 

 mon snail. Besides these feelers situated on the head, different 

 productions of the mantle are probably the seat of a finer sense of 

 touch, as the cirri round its margin in Patella and Haliotis. The 

 folded and indented fringe or circular lip in the Cephalopoda, the 

 various feelers round the mouth in Nautilus, may also be regarded 

 as organs of tact. 



Taste cannot be very highly developed, since the tongue, as we 

 stated above, is horny. Of the organ of smell nothing certain is 

 known, although in Nautilus a part, first pointed out by VALEN- 

 CIENNES, situated close to the eyes, may probably be regarded as 

 such. That, however, many molluscs possess the sense of smell 

 seems to be demonstrated by observations; thus SWAMMERDAM 

 found that vineyard snails, when he brought fresh food near them, 

 came out of their house and crept quickly towards it. According 

 to CUVIER the entire mantle that covers these animals may be 

 analogous to the pituitary or mucous membrane of the nasal cavities, 

 and thus very appropriately the organ of smell 1 . 



The auditory organ was known in the Cephalopods alone a few 

 years ago. In the cartilage of the head two small cavities are 

 found, which enclose a sacculus filled internally with a fluid, whilst 

 it is surrounded also by a fluid substance and attached to the larger 

 cavity, in which it is suspended by numerous fibrous threads. In 

 this sac, in most species, there lies a calcareous round or conical 

 little stone. The auditory nerve penetrates the saccule, and divides 

 into fine branches on its inner surface. There are neither apertures. 



also GARNER in Linn. Transact. Tom. xvn. cited above, p. 716. For the nervous 

 system of the Cephalopoda see below on this order. 



1 Lefons d'Anat. comp. 11. p. 676. BLAINVILLE, who regards the antennae of insects 

 as olfactory organs, ascribes similarly to the feelers of molluscs the capacity of smell. 

 Principes d'Anat. comp. 



