CHAPTER VII 



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



THE NERVOUS SYSTEM. 



Organs of Sense : I. Touch. 



Tactile organs, althoiiQ-h 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 specialised 

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

 Gasteropoda. 



In Cypraea (Fig. 81) these lobes, or tentaculae, are a 

 prominent feature of the animal, and also in certain genera of 

 the Trochidae (Fig. 82). In most of the carnivorous land 

 Pulmonata — e.g. Testacella, Ehytida, 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 



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