THE SENSORY RESPONSES OF CHITON 163 



of each shell plate with the same blue-green pigment ; the colored 

 scales are disposed in such a way as to form distinct, transverse 

 bands. Usually at a length of about 2.5 cm. this blue-green hue 

 upon the girdle bands is replaced by one of deep brown (burnt 

 umber); at a length of 3.5 cm. the girdle bands have usually 

 begun to fuse at the periphery, although the pale whitish patches 

 of 'scales' lying between them may persist until an advanced 

 age. This has the effect of breaking the solidity of the girdle 

 outline, when it is looked at from a little distance.^ Whatever 

 concealing effect might be attributed by some writers to the 

 coloration of the girdle in Chiton is completely defeated, however, 

 by one important fact : the chitons found in situations where this 

 condition might be of some use in the way of concealment have 

 the central area of each valve eroded (periostracum and super- 

 ficial layer of the tegmentum removed), producing along the 

 lateral region of each pleuron an isolated area, which, less eroded, 

 affords a better foothold for algae ; both when exposed to the air 

 and when under water the succession of these uneroded areas pro- 

 duces the effect of a solid, dark band, about the width of the 

 girdle, completely outlining the margin of the shell. 



The modes of pigmentation, involving blue-green and brown 

 hues, to which we have referred, are generally exhibited through- 

 out the genus Chiton. The green coloration in particular is not, 

 however, solely a matter of pigmentation. In early life the green 

 material is confined to the periostracum. Its intensity varies 

 with habitat, and ranges from a gray green to deep olive green. 

 At lengths above 3.5 cm., however, the chiton becomes covered 

 with a thin coating of dark green algae. The character and 

 extent of this coating varies with habitat. In some cases the 

 green pigment may completely disappear, producing chitons of 

 the 'variety' assimilis of Reeve. In all cases there is a pronounced 

 homochromic element in the general appearance of the chitons. 



* A small species of Onchidella found in this habitat is (like many other On- 

 chidellas) also pigmented according to this general plan. The free margin of its 

 mantle is colored with a dark brown pigment, interrupted at regular intervals 

 by light patches containing glands. The dark pigment forms denser, but irregu- 

 lar, patches upon the dorsum; the whole effect is of a dense, but moderately 

 homochromic, oval mass surrounded by a banded 'girdle' — but all on a very 

 minute scale, as the creature is only 2.5 mm. long. 



