Chap. XV.] THE SNAIL. 299 



posterior tentacle. If in a vigorous and healthy snail the pos- 

 terior tentacle be removed, the eye is regenerated. In some 

 cases it has been found that several perfect eyes have regener- 

 ated in the same tentacle. 



The auditory organ is a minute sac (Fig. 86, A. and B.), with 

 which the auditory nerve is connected. The sac contains a 

 number of otoliths aggregated into a mass. 



With regard to the sense of smell there has been much 

 discussion. The anterior tentacle is often spoken of as olfactory. 

 Near its end is a large ganglion from which fibres pass to the 

 epithelium, in which there are said to be developed sensory 

 knobs. Snails, however, in -which these tentacles have been 

 removed by excision, are apparently still possessed of a sense 

 of smell. The lobate processes round the mouth have been 

 regarded as the seat of olfactory sensation. Eecent investigation, 

 however, seems to point to the glandular rather than the sensory 

 nature of these organs. Finally it has recently been contended 

 that the pedal gland is the olfactory organ. Some of the cells 

 are glandular and mucus-secreting. Others are deeper-seated 

 and oval, and give off a delicate rod which enlarges at its free 

 end into a ciliated knob. These are regarded as sensory. Thus 

 it is maintained that we have in this organ the three necessary 

 factors of an olfactory organ, the presence of a layer of sensory 

 cells, the entrance of air, and the addition of a secretion from a 

 glandular organ. 



With regard to the seat of a sense of taste little or nothing 

 definite is known. 



The whole body is sensitive to touch, but in the tentacles 

 this sensitiveness is much more delicate than over the general 

 surface. The nerve that proceeds to the posterior tentacle 

 bifurcates within that organ. One branch goes to the eye. The 

 other passes to a ganglion, from which fine threads pass to 

 a highly sensitive prominence at the end of the tentacle. It is 

 very probable that we have here a delicate tactile organ. 



The Organs of Generation. The snail is hermaphrodite, 

 male and female organs being found in the same individual. 



