Experiments were carried out on snails of three age groups: Snails of the first age group were taken from the nest and were under 1 month old, those of the second age group were 4.0-4.5 months old, and in the third age group were sexually mature snails (5-8 years). Snails of the first two age groups were reared under laboratory conditions in a terrarium. Responses to tactile and photic stimulation were studied under conditions allowing relative mobility of the animal, but in a position convenient for stimulation [i]. Hairs calibrated by weight (9, 25, and 90 mg) were used for tactile stimulation. A stimulus was applied in the region of the head at the base of the left ommatophore with a frequency of once every 20 sec. Stimulation ceased after no defensive reflex was observed to three consecutive stimuli. Under the same conditions the snail's responses to extinction of light for 3 sec were investigated. Responses to vestibular stimulation were investigated in a simplified version of the apparatus described by Humphrey [6]. The snail was placed in the middle of a previously moistened light wooden platform supported by a rubber band. The platform oscillated when a weight fell on its edge from a certain height. The extrastimulus was a powerful jet of air, which led to retraction of the anterior part of the snail's body. The contractile response of the ommatophores was recorded visually and assessed as a percentage of the maximal degree of retraction of the ommatophore. The stimulus was applied at a frequency of once every 20 sec. The results of the study of changes in the response of the snail to repetitive tactile stimulation were obtained in 76 experiments on ii snails of the first age group, 19 of the second, andl5 mature snails. Defensive responses to repetitive tactile stimulation on mature snails were characterized by sensitization, manifested as an increase in magnitude of the response to the second or third stimulus, if the response of the ommatophores to the first stimulus in the series was not maximal, or as retraction of the head and the whole anterior part of the trunk, if the ommatophores contracted completely in response to the very first stimulus. During further stimulation the magnitude of the response fell to zero by the 15th30th presentation of the tactile stimulus, depending on its weight (Fig. i). The time course of the change in response of the ommatophores of snails of the second age group was similar in principle and differed only in its more rapid decline to zero and in the fact that the snail responded to repetitive tactile stimulation with a weight of 25 and 90 mg by complete retraction of the body into the shell by the 5th-10th presentation of the stimulus. In snails of the first age group sensitization of the response was not observed: The ommatophores no longer contracted to the 2nd-3rd stimulus in the series, and the shape of the curve in this case was independent of stimulus intensity. After interruption of stimulation for 15 min the response to tactile stimulation was restored, but toa different degree, in the snails of all age groups. Whereas a snail of the first age group usually responded only to the first stimulus in the second series by a weak contraction of the tentacles or did not respond at all (Fig. i, Table i), the time course of the change in response of snails of the 3rd and 2nd groups continued to be in the previous form. The results of investigation of the change in response of the snail to repetitive vestibular stimulation were obtained in 45 experiments on 15 snails of the first age group, seven