254 LESLIE B. AREY AND W. J. CROZIER 



intensity, not by an increase. The dorsal surface of the girdle 

 (scales) is also sensitive to light — characteristically to a decrease 

 of light intensity, also to the constant intensity of light, and 

 to a sudden increase in light intensity, provided this intensity be 

 great. The soft ventral surfaces are sensitive to light. The 

 periphery of the girdle is the ventral part most sensitive to 

 shading. 



The superficial soft tissues of Chiton are open to chemical 

 activation, to stimulation by abnormal osmotic pressures, and 

 by 'irritants.' 



Evidence has been secured, through the study especially of 

 the topographic distribution of the various types of excitability, 

 that tactile receptors, photic receptors, and chemoreceptors are 

 physiologically distinct. There is no clear evidence of sensitivity 

 to heat; that to cold is less doubtful. 



There is a pronounced tendency for the animal to come to rest 

 in positions avoiding uneven tensions in the musculature. This 

 is responsible for the precise negative geotropism exhibited by 

 Chiton. This mollusk is not sensitive to vibratory mechanical 

 disturbances. 



b. This brings us to the consideration of one of the most un- 

 settled problems in sensory physiology : In what manner is differ- 

 ential irritability determined? The immediate receptors of ex- 

 citation in metazoans above the sponges are cells which function 

 primarily as detectors and transmitters of disturbances in the 

 energies of the environment. In a general way it is true that all 

 forms of protoplasm are capable of being changed (activated) by 

 light, heat, cold, pressure, chemical agents, and so forth. Con- 

 siderations of this order, which hold also in certain cases upon 

 the quantitative side (as in the action of chemical agents), have 

 been responsible for the view that irritability' is a generahzed 

 property of living matter, best studied in uni- (or non-) cellular 

 organisms. It does not seem probable that this conception can 

 at present be of any great help ; in spirit it is deductive, whereas 

 the manifestations of irritability (e.g., in the diversified taste 

 receptors of the human tongue) are manifold, specific, and must 

 be investigated in a more purely inductive manner. Neverthe- 



