Ill 



THE KILLIFISH 



The killifishj Fundulus heteroclitus, is a small fish some 

 three to four inches in length, found in the coastal and 

 estuarial waters of the eastern shores of the United 

 States from Maine to Texas. It can be transferred di- 

 rectly from fresh water to salt water and back again 

 with impunity and in other respects as well it is very 

 hardy. In consequence it is a favorite live-bait for win- 

 ter fishermen and a very convenient animal for the 

 experimentalist. Its skin is abundantly supplied with 

 melanophores, the larger of which are often associated 

 with iridocytes. It also possesses a large number of 

 dermal xanthophores and a few local guanophores (Odi- 

 orne, 1933). Under appropriate circumstances depend- 

 ent for the most part on the background, Fundulus may 

 assume a pinkish, bluish, greenish, yellowish, or steel- 

 black tone, or pass over into a pale gray or almost pearly 

 white stage (Connolly, 1925). Some of these phases, 

 such as the pinkish one, are reached only very slowly 

 and may require days or even weeks for full accomplish- 

 ment. Others, like the changes from pale to dark and 

 the reverse, may be brought about in a few minutes. 

 These latter changes, as might be expected, are depend- 

 ent upon the action of the melanophores and have been 

 studied much more generally than the others. The 

 change from pale to dark and the reverse take place with 

 remarkable certainty and rapidity (Fig. 17). When a 

 pale Fundulus in a white-walled, illuminated vessel is 

 transferred to one with black walls, the fish will change 

 from its original pale tint to a dark one in a little less 

 than a minute, though the full completion of this change 



