202 BULLETIN OF THE BUREAU OF FISHERIES.. 



animal became almost fully adjusted in the time it required the body to cross the line. 

 As the fish went from the black to the white it appeared as though the black chroma- 

 tophores were being rapidly buried in a white substance; and, as it returned to the 

 black, as though they were being rapidly uncovered. 



At 10.30 a. m. this specimen came to rest with the head on the white and the body 

 and tail on the black, in such a position that the longitudinal axis made an angle of 

 about 30 degrees with the line between the two halves of the aquarium. One eye was 

 2 cm. from this Une and the other farther away. The skin' immediately became much 

 lighter. The fish remained in this position until 12.45 p. m. At this time it was much 

 darker than when fully adapted to the white and much lighter than when fully adapted 

 to the black bottom. It was gray, somewhat mottled but not contrastive, and the same 

 over the entire pigmented surface. At 12.45 P- m- ^^^ specimen moved forward 15 cm. 

 so that it was entirely on the white. It at once became much whiter, and soon appeared 

 maximum white. At 1.15 p. m. it returned to the black bottom and at once became 

 much darker. At 1.50 p. m., when the observations were closed, it was still on the black 

 bottom and it appeared maximum black, having, as usual when in this condition, a num- 

 ber of conspicuous white spots scattered over the surface. 



During the course of these observations, while this specimen was on the white bottom, 

 a large dark four spot, 20 cm. long, several times swam slowly toward it. Each time, as 

 the four spot approached a point approximately 10 cm. from the anterior end of the 

 specimen of Paralichthys under observation, this specimen suddenly became much darker. 

 Whether this change in shade was due to the darkness of the four spot and the consequent 

 reduction in reflected light, or the excitement caused by its presence, I am unable to say, 

 but other observations show that dark objects at a distance sometimes do have an effect 

 owing to the reduction of light. Similar phenomena were observed in a considerable 

 number of other specimens. Details in reference to one of these follow: 



A specimen of P. albiguttus 21 cm. long was kept for several weeks in a large quarium, 

 one half of which was black and the other half yellowish white. On August 19 and 20 

 it was frequently transferred from the black to the white bottom and vice versa. Adapta- 

 tion to black and white, at the end of this period, was found to be rapid. On August 21, 

 2.30 p. m., it was found on the black bottom fully adapted. It was at once put into water 

 about 4 cm. deep in a glass crystallizing dish 50 cm. in diameter. This dish was placed in 

 water in strong diffused light, on a sheet of bristol board half black and half white, and 

 so adjusted as to continuously keep the anterior half of the animal over the white and 

 the posterior half over the black bottom. The entire surface of the fish became light 

 grayish white almost at once, and it soon appeared to be maximum white. The fish was 

 held in this position until 4.55, photographed (fig. 44), and then rotated until the 

 anterior end was on the black. 



At 7 p. m. it was nearly maximum dark. At this time it was set free in the dish and 

 left. At 10 p. m. it was found with the anterior end on the white and the entire surface 

 appeared to be, if anything, whiter than it had been at any time previous, although the 

 light was so weak that during most of this time only that part of the fish which differed 

 in shade from the background could be seen. 



The following morning it was still maximum white. At 8 a. m. the anterior end 

 was put and held on black. At 10 a. m. the entire surface was nearly maximum black. 

 It was then turned until the longitudinal axis was parallel with the dividing line in 



