CHAPTER YII 



ORGANS OF SENSE: TOUCH, SIGHT, SMELL, HEARING — THE 

 EOOT — THE NERVOUS SYSTEM 



Organs of Sense : I. Touch 



Tactile organs, although occurring in some of the Mollusca, 

 do not appear to attain special or marked development, except 

 in a few cases. The whole surface of the skin, and particularly 

 of the foot, is very sensitive to the slightest impression. Nearly 

 all Gasteropoda are furnished with at least two cephalic tentacles, 

 projecting like horns from each side of the fore part of the head. 

 At or near the base of these are generally situated the eyes. 

 In the Helicidae the eyes are situated, not at the base, but at 

 the apex of the tentacles, and in that case — except in Vertigo — 

 a second pair of shorter tentacles appears beneath the longer 

 pair. It frequently happens that several senses are centred in 

 a single organ ; thus the upper tentacles of snails not only carry 

 the eyes and serve to a certain extent as tactile organs, but they 

 also carry the organs of smell. 



The edges of the mantle, which are sometimes special- 

 ised into lobes, appear to be keenly sensitive to touch in all 

 Gasteropoda. 



In Cypraea (Fig. 81) these lobes, or ten taeniae, are a promi- 

 nent feature of the animal, and also in certain genera of the Tro- 

 chidae (Fig. 82). In most of the carnivorous land Pulmonata — 

 e.g. Testacella, Rhytida^ Ennea — there are developed, under the 

 lower pair of tentacles, and close to the mouth, large labial 

 palps or feelers. These are connected with the cerebral ganglion 

 by a very large nerve, and may therefore be supposed to be of 

 extreme sensitiveness. In some of the large carnivorous forms 



VOL. Ill 177 N 



