THE SENSORY KESPONSES OF CHITON 229 



/3. One end of the body illuminated. Similar tests were made 

 in which either the anterior or the posterior end of the animal 

 was illuminated. Numerous experiments of this sort were also 

 made in the field. In brief, chitons upon sunlit rocks were found 

 to move into the light when either the anterior or the posterior 

 end had been shaded. The locomotion subsequent to illumina- 

 tion of the posterior half only, by means of bright sunlight, was 

 in a posterior direction; usually, before the chiton had moved 

 completely into the light, it executed a turning movement. The 

 readiness with which backward creeping, for distances of several 

 centimeters, may be resorted to, is worthy of remark. 



7. Analysis. The predominating movement in these tests is 

 photopositive. The peculiar variations observed remain to be 

 explained. This can be done, as in the case of orientation by 

 general illumination, through consideration of the size and habitat 

 of the individuals and of the character of their periostracum and 

 tegmenta. This analysis agrees in its results with that pre- 

 viously given. The small chitons, less than 2 cm. in length, move 

 into the shadow when their surface is half illuminated. They 

 creep backward with greater readiness than do the large ones. 



These experiments show that with light approximately ver- 

 tical in direction the effect of partial illumination of the body is 

 such as to parallel completely the result in orientation to hori- 

 zontal light. 



2. Differential sensitivity 



a. Shading. An attached chiton, undisturbed and at rest, 

 tends to lift the girdle from the substratum, either along its 

 whole circumference or else in one or more local areas. If. a 

 raised portion of the girdle be shaded, there results a relatively 

 quick and smooth lowering of the girdle to the rock or other 

 surface. This response occupies about two seconds; its vigor 

 depends upon the original distance of the girdle from the sub- 

 stratum. After a little time (about ten seconds) the part con- 

 tracted is relaxed to its original condition. The speed of re- 

 action and the time for recovery vary considerably in different 

 animals. In some cases a single stimulation resulted in the 



THB JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 29, NO. 2 



