THE SENSORY RESPONSES OF CHITON 227 



These six specimens illustrate a correlation which is universal 

 in our experience: the youngest chitons are found in dark- 

 situations, the older ones in the light; these two groups are, 

 respectively, photonegative and photopositive to ordinary sun- 

 light; animals of intermediate size are photopositive to weak 

 light, photonegative to stronger light, and the character of their 

 normal photic environment (typically, in horizontal crevices 

 near the mouth of caves, and in other non-brilliantly illuminated 

 spots) is completely correlated with this behavior. There is 

 usually a region of intensities of ordinary daylight within which 

 an animal of intermediate size may be either photopositive or 

 photonegative in different tests — a region of seeming indifference 

 to light. The largest chitons are usually quite indifferent to 

 weak, diffused light. The actual distribution of the chitons of 

 different sizes in the field shows in a most convincing manner 

 that this differentiation in photic responses is not a matter of 

 adaptation to environmental circumstances, but is on the con- 

 trary based upon structural changes determined with advancing 

 age. Note, for example, the following instances in which an 

 individual photonegative to sunlight (as found by test) occurred 

 on a shore where no deep caves were available, nor any large 

 stones under which it might hide. 



VI. 111. (Apl. 4, 1918). North shore of Hawkins Island; a more 

 or less horizontal shelf of rock, 1 foot beneath high water mark; nine 

 chitons in a closely compacted group, in the zone of Modiolus and 

 barnacles. Eight of the chitons with eroded valves, forming a fairly 

 close match with the color of the exposed rock; in sunlight; these 

 chitons 6.5 to 8.8 cm. in length. One chiton, however, was a d^, 4.5 

 cm. in length, the valves greenish, very slightly eroded; it was located 

 under another individual (cf, 6.8 cm. long), which completely con- 

 cealed and sheltered it from the light. 



VI. 140 (Apl. 22, 1918). Northwest shore of Marshall Island. 

 In a small pocket in the rock four chitons of 7.2 to 8.7 cm. were found; 

 of these the shells were eroded and bleached. Under one of them a 

 fifth specimen, 2.8 cm. long, blue-green in color, valves uneroded. 



h. Results of partial illumination of the body. a. One side of the 

 body illuminated. Chitons were adjusted in the apparatus 

 shown diagrammatically in figure 13; they were so situated, 

 directly under the vertical partition, that either the right or left 



