THE KILLIFISH 23 



black. Another feature that should be noted in passing 

 is that a Fundulus from which the eyes have been re- 

 moved is pale in darkness, as normal individuals are, 

 but moderately dark in bright illumination. This dark- 

 ening of blinded fishes in illuminated vessels occurs to 

 an equal degree whether these vessels have black walls 

 or white walls (Parker and Lanchner, 1922). Such en- 

 vironmental differences are responded to, as might be 

 expected, only when the eyes of the fish are functional. 

 The general color conditions just described for Fun- 

 dulus are exhibited by this fish the year round, but 

 during the breeding season, the height of which is toward 

 the end of June, all color responses are greatly intensi- 

 fied, especially in the males. In fact at this time of 

 year the male is readily distinguished by a dark eye-like 

 mark on its dorsal fin, a nuptial secondary sex-character, 

 which is formed by an aggregation of melanophores in 





Fig. 18. Four enlarged views of the dorsal fin of the killifish, 

 Fundulus, showing the nuptial mark in the male (1, 2) and its 

 absence in the female (3, 4). Figures 1 and 3 represent the dark 

 phase and figures 2 and 4 the pale phase of this fish. Parker and 

 Brower, Biol. Bull., 1935, 68, 5. 



