538 W. H. LONGLEY 



it happens to attach itself to one surface or another of large 

 fishes to which it is commonly found adhering. Its essential 

 lack of countershading is therefore correlated with the fact that 

 it maintains no constant position with reference to the source 

 of light. Fierasfer lives within the cloaca of large holothurians 

 upon the reef flats, and not only has no countershading, but has 

 even lost its external pigment. Fishes which habitually live 

 in dim light, e.g., the glass-eyed snapper (Priacanthus cruentatus) 

 are only slightly countershaded, and the same is true of such 

 fishes as the Chaetodontidae, whose thickness is little in compari- 

 son with their depth, and whose sides as a consequence are 

 nearly vertical. 



Ob\dously all these facts and many more, such as the reversed 

 countershading of Agalena naevia, a common spider which hangs 

 in its web ventral side uppermost, that of the caterpillar of Auto- 

 meris io which feeds in an inverted position, and the absence of 

 external pigment in the crab (Pinnotheres) found in the mantle 

 cavity of the oyster, may be referred to one system, which in- 

 volves the production of dark pigment in any region in direct 

 proportion to the average intensity of the illumination of that 

 part. They may be equally well explained by supposing that 

 the pigmentation observed is the effect of insolation, or the result 

 of selection directed toward the development of inconspicuousness. 



The second hypothesis may unquestionably be attacked from 

 the vantage ground of Cunningham's ('91, '97) observations and 

 experiments, which show that, if flounders be illuminated from 

 beneath, pigment will develop upon their lower sides. To argue 

 that this is scarcely a fair test, since the lower side of a flounder 

 is not a true ventral side really lacking dark pigment through 

 unknown generations, and that these experiments show the power 

 of light to recall, but not to induce the creation of pigment anew 

 in tissues in which it had not appeared during the racial history, 

 would be of little avail. But the hypothesis that countershading 

 is the direct effect of illumination is after all more vulnerable 

 than the other. We may admit that we do not understand the 

 change in the remora's pigmentation; we do not know that it 

 has been effected through natural selection, but there is an inter- 



