THE SENSORY RESPONSES OF CHITON 161 



restricted, C. tuberculatus is relatively abundant. During late 

 summer and autumn, young individuals, less than 1 cm. in length, 

 are to be found beneath low-tide level on the under surfaces of 

 stones, glass bottles, and other more or less smooth objects. In 

 spring and summer the youngest (smallest) specimens obtainable 

 are characteristically found under small, flat stones on sheltered 

 beaches, at the upper limit of the tides. Larger individuals are 

 not usually found in this type of habitat. The age at which 

 sexual maturity comes about (Crozier, '19), namely, when a 

 length of about 3.5 cm. is attained, is correlated in a general way 

 with the occurrence of the chitons further down the shore, under 

 larger stones or in crevices in the walls of caves. Still larger 

 chitons, 5 to 7 cm. long, commonly occur freely exposed upon 

 sunlit rocks, while those of maximal size (8 to 9 cm.), are rarely 

 found concealed in dark places. 



The youngest chitons, which as a rule live under small stones 

 and are seldom if at all found exposed to bright sunlight, are of 

 a general light greenish cast. The periostracum is lustrous and 

 perfectly intact (uneroded) . Very rarely indeed are adventitious 

 organisms (barnacles, Spirorbis, or algae) found lodged upon 

 them. In many, in fact in most, places their coloration matches 

 decidedly the tint of their surroundings among the smooth, 

 greenish under surfaces of the stones occurring at the upper 

 reach of the tide. Moreover, the pigmentation is not 'solid,' but 

 is broken up by small light and dark blotches. The homochro- 

 micity of the pigmentation is not, however, in all cases perfect. 



In coloration and general appearance C. tuberculatus assumes 

 the aspect more characteristic of the species at a slightly later 

 period. Animals 5 to 7 cm. long (fig. 1) correspond in form and 

 markings with Haller's figure of C. squamosus, well known 

 through its reproduction in many zoological texts. The close 

 relationship of C. tuberculatus and C. squamosus — which for 

 our purposes is fortunate because the anatomy of the latter and 

 of its relatives is well understood — is shown by the fact that until 

 comparatively recent times these species were considered iden- 

 tical by conchologists. At least two distinct pigment materials, 

 and perhaps a third, enter into the general coloration of C. tuber- 



