230 LESLIE B. AREY AND W. J. CROZIER 



girdle being closely applied to the substratum for a long time 

 (save at the anterior and posterior ends, involved in respiration) . 

 In other instances five to nine shadings and responses were 

 obtained, after which the response grew weaker until it finally 

 disappeared. When the response became very weak, or imper- 

 ceptible to a single shading, two or more shadings in fairly rapid 

 succession were still effective in producing a reaction from the 

 girdle. Associated with depression of the girdle is a pronounced 

 contraction of the gills in animals shaded dorsally. 



Chitons resting upon their dorsal surface and shaded ventrally 

 gave also a pronounced response. Some individuals were very 

 sensitive, coming from a fully extended condition to complete 

 rolled-up closure as the result of a slight decrease in the illumi- 

 nation; if already partially rolled up, a general shading induced 

 still further, but still incomplete, curling of the body, after which 

 the chiton returned to its original condition. The response 

 became fatigued after seven to sixteen trials. In good reactions 

 of this type, five seconds were required for the curling up of the 

 shell, fifteen seconds for its recovery. To successive shadings 

 the curling-up process becomes more complete ; the time required 

 for this effect varies directly with the size of the animal, as seen 

 in the following notes (table 3) ; some animals are decidedly less 

 reactive in this way than others, however. In Holothuria 

 (Crozier, '14) conditions of similar import are known. 



The reaction to shading of the ventral surface comprises 

 longitudinal bending, contractive movements of foot and head, 

 inward curving of the edge of the girdle, and contractions of the 

 gills. The time occupied by the response of the ctenidia ('reac- 

 tion time') to a single shading at the posterior end of the animal 

 is also proportional to the length of the Chiton (table 3). 



The response to shading is due to a decrease in the intensity 

 of light in the visible region of wave lengths. The interposition 

 of a glass plate between a chiton and the source of light led to no 

 reaction, and the shading response was easily elicited through a 

 considerable thickness of glass. Glass ray filters transmitting 

 restricted portions of the spectrum were placed between chitons 

 and a source of sunlight (which induced shading responses), 



