THE SENSORY RESPONSES OF CHITON 237 



animal will orient precisely away from the light, and then move 

 caudad toward the light for a distance of several centimeters. 

 Close inspection usually makes it evident in these cases that the 

 posterior valve is more deeply eroded than the anterior valves. 

 Its sensitivity is therefore less. Similarly, a few animals were 

 noted in which photopositive orientation was succeeded by 

 backward creeping, i.e., away from the light, owing presumably 

 to the higher sensitivity of the anterior valve. There is evident 

 in these reactions a type of local response, rather than of unitary 

 behavior, which is highly instructive and deserves further 

 study. 



In Ischnochiton the behavior above referred to is somewhat 

 similar to that just described, as this amphineuran creeps poste- 

 riorly with considerable freedom. Horizontal light parallel to 

 the long axis, incident upon the anterior end, induces the animal 

 frequently to creep backward for several centimeters. 



In chitons relatively uneroded or evenly eroded the method 

 of orientation is direct, diagrammatic, as it is in most mollusks. 

 In a large individual the process is so slowly carried out, unless 

 very bright light is used, that its details may be carefully ex- 

 amined. The results of such examination are not merely con- 

 sistent with the theory of Loeb regarding animal phototropism, 

 but furnish valuable evidence in support of this view. The 

 behavior of Ischnochiton and of unevenly eroded large individuals 

 of chiton is of particular importance in this connection. 



5. Bionomic correlations involving photic behavior 



Many writers have considered the photic reactions of animals 

 in the light of their normal conditions of life, coming usually to 

 the conclusion that these reactions are highly adaptive, and 

 hence that they have been determined through natural selection 

 (Mast, '11, chap. 13). In spite of its traditional sanctity, the 

 presentation of this view involves an inversion of logic, a veri- 

 table somersaulting, which is to the last degree unconvincing. 

 That the modes of response which specific organisms are found 

 to exhibit are individually correlated in an appropriate way with 



