THE SENSORY RESPONSES OF CHITON 239 



varying relative importance. The general feature which is the 

 most significant, however, is the correlation of slight exposure 

 with incipient erosion. A study of over 1500 individuals has 

 shown the invariable completeness of this correlation. The 

 amount of exposure is, in part at least, determined in a purely 

 mechanical way. The smallest chitons, dwelling at the very 

 upper limit of the tide, under loosely piled small stones, become 

 after a year or two too big to fit into the crevices there provided. 

 The food supply is also insufficient. They therefore come to 

 inhabit stations further below high-water mark. The occurrence 

 of other organisms is here more abundant, and there are addi- 

 tional factors making for more ready erosion. 



In this way the history of a chiton can be followed in a fashion 

 which shows that although its habitat is determined by its be- 

 havior, the reverse is not apparently the case. Hence the logical 

 inversion to which reference was made in the first paragraph of 

 this section. It is also evident that the modifications in Chiton's 

 behavior and appearance, its occurrence in groups, and the 

 probably advantageous correlations in this way resulting, are 

 determined in a catenary manner. The original position (habitat) 

 of the young individual is determined by the tropisms of the 

 larva, in their turn determined by the inherited chemical com- 

 position of the egg. The lack of active wandering movements 

 (p. 178; Plate, 'Ola, p. 509) in the older individuals is important, 

 because opportunity is in this way made for the full operation 

 of the influences in particular circumscribed habitats, and thus 

 for the development of homochromic elements in the coloration 

 of the chitons and for concomittant phases of progressive erosion 

 of the shell. 



The chitons as a whole are known to be photosensitive and to 

 reside in general in dark situations (Cooke, '95, p. 400). Heath 

 ('99) observed that a number of chitons were nocturnal in their 

 habit, "withdrawing into some shaded position upon the approach 

 of day," some species remaining out on their feeding grounds 

 "only when the day is foggy or dark." The larva of Ischno- 

 chiton magdalenensis is positively heliotropic, the adult negative 

 (Heath, '99). It would be of some interest to study a variety of 



