222 LESLIE B, AREY AND W. J. CROZIER 



various parts of Chiton under the action of these agents follows 

 the relative orders : 



dorsal mantle > foot, palp, ctenidia, for heat; 

 foot, palp > ctenidia, for cold. 



To tactile excitation, the order of reactiveness is slightly dif- 

 ferent (see p. 199), but there exists no adequate criterion for the 

 comparison of the relative sensitivity of the several parts to heat. 

 Only in the case of the dorsal mantle, between the plates, does it 

 appear that thermal sensitivity is relatively enhanced as compared 

 with tactile, since the high-temperature threshold (37°) seems to be 

 lower than for other regions (40°) which are superior to the inter- 

 tegmental mantle in tactile reactivity. The amplitude and vigor 

 of the responses from this region are comparatively slight, how- 

 ever, and little emphasis can be put upon this result. On the 

 basis of their relative distribution, thermal and tactile receptivity 

 cannot be clearly separated. 



By differential exhaustion an apparent separation of this kind 

 can, nevertheless, be effected. When water at 10°C. was poured, 

 in 1 cc. portions, from a pipette several times in succession over 

 the anterior ctenidia of a chiton in water at 24°, the animal ceased 

 to respond after the fourth treatment; six applications of cool 

 water were made at intervals of three minutes. Very weak tactile 

 responses were then obtainable from the affected ctenidia, al- 

 though they still did not respond to water at 10°C. In attempt- 

 ing to differentiate between 'heat' and tactile responses, this 

 method of attack fails completely, since, as we have described, 

 when chiton is placed in water at 38°, its general tactile reactivity 

 was perceptibly increased, and much more conspicuously so im- 

 mediately after immersion in water at 40°, although tactile re- 

 activity gradually decreases after a few minutes' exposure to this 

 latter temperature. As a consequence of this condition, we are 

 not warranted in speaking either of the separateness or the sen- 

 sory identity of 'heat' and tactile effects, even though the surface 

 of the foot and the ctenidia did almost cease to respond to touch 

 after they had been exposed to four local treatments, in air, with 

 1 to 2 cc. of water at 40° at intervals of two minutes. 



