THE SENSOKY RESPONSES OF CHITON 181 



to be considered. They concern phenomena of coloration, re- 

 production, determination of 'choice' of habitat, and similar 

 features, comprising some of the things which involve explanation 

 in terms of the animal's sensory physiology. In the matter of 

 coloration, for example, the chitons in " general exhibit homo- 

 chromic ('concealing') characteristics which are commonly of 

 some precision (e.g., in Cryptochiton, Heath, '05 b, p. 213, 

 and in other genera which we have observed; cf. also Plate, 

 1901 a, p. 376). In C. tuberculatus this homochromic correla- 

 tion is decidedly evident — in most cases it is unmistakable. It 

 involves several pigments, their mode of distribution, the t)ver- 

 growth of the shell by algae, barnacles, etc., a shifting of the chiton 

 during growth to stations further below high-water level, the 

 erosion of the valves, and a further shifting to more exposed 

 habitats, with corresponding changes in the appearnace of the 

 creature. There is little reason to doubt that in the later growth 

 of the chitons (three to four years old) conditions of food supply 

 directly determine through the course of metabolism the char- 

 acter of the. pigmentation displayed in the periostracum. The 

 most fundamental factor concerned in the changing habits and 

 appearance of chiton with advancing years, however, is its move- 

 ment into more illuminated areas. The whole problem of its 

 bionomic correlations becomes, from this standpoint, somewhat 

 more directly open to attack. In general, it is not: How are the 

 bodily processes kept going by the aid of movements? and, How 

 does it happen that the movements are of such a character as to 

 keep the processes going? (Jennings, '07, p. 57), but rather: 

 What is the relation between the sensory capacities which deter- 

 mine and direct the bodily movements, on the one hand, and on 

 the other hand the way in which the bodily processes are found 

 actually to be going? 



Naturalists have long been content to assign a given ' reaction' 

 to some one or another of the categories of adaptation, and to 

 rest satisfied that progress had thus been made in explaining it. 

 No progress can be made in this way. Neither are we greatly 

 helped by placing the responsibility for the adaptation in a 

 general way upon the environment. The situations requiring 



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



