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CHAPTER VII
ORGANS OF SENSE: TOUCH, SIGHT, SMELL, HEARING—THE FOOT—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 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, 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 (Glandina, Aerope, compare Fig. [21], p. 54) these palpae are of great size, and curl upwards like an enormous pair of moustaches. When a Glandina seizes its prey, the palpae (see Fig. [83]) appear to enfold it and draw it in towards the mouth.