526 ADAPTATIONS TO PHOTIC QUALITY 



a chance to affect it by expanding. Both the expansion and the contrac- 

 tion of the pigment mass appear to be active processes — neither is com- 

 parable with the relaxation of a muscle. 



Chromatophores are of several types. Most widespread of all is the 

 'melanophore', containing the dark brown, almost black pigment melanin. 

 The predominance of melanophores is largely responsible for the fact that 

 dermal changes of shade, in an achromatic sense, are more widespread 

 among species and more conspicuous in individuals than are changes of 

 hue. This predominance probably indicates antiquity. Other types of 

 chromatophores would seem to be newer inventions. Some of them oper- 

 ate quite differently from melanophores. In a particular species, one type 

 may be changeable and another not; and in some animals (lizards, 

 snakes) they are all quite inert so far as we can tell. 



Some colored chromatophores, generically called lipophores because 

 their pigments are fat-soluble carotenoids, take their special names from 

 their colors : erythrophores (red) , xanthophores (yellow) , xantholeuco- 

 phores (changeable from yellow to white) and so on. A third class is 

 comprised by the iridocytes, which may be inert or active, free or associ- 

 ated closely with other chromatophores to form iridosomes. The pigment 

 in iridocytes is the familiar guanin, which may give the cell a white or sil- 

 very color, or even produce an enamel-like yellow, blue, or green depend- 

 ing upon the way in which the platelets of guanin operate to produce in- 

 terference between the wavelengths of light they reflect. A single iridocyte 

 may, as in Fundulus parvipinnis, scamper through green, orange, yellow, 

 and red phases in successive moments. 



*PhysiologicaV and ^Morphological' Chromatophoral Changes 



Chromatophoral changes may have little to do with illumination, or 

 they may closely adapt an animal to the shade of its surroundings, to the 

 color of the background, or to both. In some species (as certain flound- 

 ers) even the pattern can be roughly matched, as Sumner first showed 

 in 1911. The fishes take on large blotches when over a coarse polka-dot 

 pattern, small spots when on a small-dotted background. These rapid, 

 transitory changes (not of pattern, however) were of course known to 

 the ancients, and were described for the chameleon and invertebrates 

 (cephalopod molluscs) by Aristotle. As early as 1882 Flemming sug- 

 gested, on the basis of his experiments with salamander larvae, that the 

 actual number of chromatophores could be influenced by the surround- 

 ings of the animal. In 1909, Secerov coined some terms to express the 

 distinction which his work on a fish (Barbatula barbatula) led him to 



