EVOLUTION OF ASYMMETRY 21 



the " upper " eye is nearly on top of the head, and the bUnd side of the head and body 

 is pigmented, althou.gh the coloration is paler than on the ocular surface. The large, 

 symmetrical mouth, strong teeth, elongate body, slender caudal peduncle and powerful 

 lunate tail are all characters which denote an active piscivorous fish, which probably 

 spends the greater part of its time away from the sea floor. There can be little doubt 

 that the pigmentation of the blind side, like the plumpness of the body and the position 

 of the ' ' upper ' ' eye, has been secondarily acquired by a fish which has to a great extent 

 forsaken the normal habits of the group, a view which is borne out by a study of its 

 development. The pelagic larval stage, as in other Flatfishes, is pigmented on both 

 sides, although the coloration of the right side (i. e. the future ocular side) is darker. 

 After passing into the bottom stage the pigment of the blind side gradually disappears, 

 and this surface becomes quite white to the naked eye ; later on, pigment is again 

 developed on the blind side (Jensen, 1925, p. 10). 



The powers possessed by Flatfishes of changing their coloration to simulate the 

 ground on which they lie are well known. The view of Cunningham (i8yo, pp. iio- 

 113)^ that the colour changes are due to the action of light, and have nothing to do 

 with the colour of the ground, does not meet with much favour among other investi- 

 gators, and it is now generally believed that a definite relation exists between the 

 coloration of the ocular surface of the fish and that of the ground on which it lies. 



Sumner (191 1) has conducted a series of interesting experiments on the Mediter- 

 ranean Bolhiis podas, in which living individuals were placed successively on various 

 backgrounds, consisting of patterns of black and white squares, black and white 

 circles and so on. It was found that a fish placed on a background of this nature 

 responded more slowly than when placed on more accustomed grounds (sand, pebbles, 

 gravel, mud, etc.), but did succeed in simulating the artificial background to a 

 remarkable degree. The skin patterns were found to \'ary not only with the relative 

 amounts of black and white in the l)ackground, but also with the degree of subdivision 

 of the areas of the latter. Further, it was found that with repetition of the experiment 

 on the same individual the rate at which the fish was able to simulate its surroundings 

 was considerably accelerated. Sumner points out that the capacity of the fish to 

 adapt itself to different backgrounds is definitely limited to the black, grey and brown 

 of its ordinary' environment, and brilliantly coloured grounds seemed to be beyond 

 its powers of imitation. Moreover, the species is provided with permanent spots 

 and other markings due to the special grouping of the chromatophores in the skin, 

 and, although these vary in relative intensity and may even disappear altogether, 

 they always occupy the same position when present. 



The work of Mast (igi6) and Kuntz (IQ18) on Paralichthys and Ancylopselta is 

 even more striking. Mast found that Paralichthys was able to assume various colours 

 which corresponded very closely with the background on which the fish was placed, 

 although some shades proved more difficult to simulate than others. Reds of various 

 tints were found to be simulated much less accurately than whites, greys, blacks, 

 browns, greens, blues, yellows or oranges ; further, on yellow or brown backgrounds 

 the responses were much more rapid than on reds, greens or blues, on wliich it might 

 take two or three months for the fish to complete the colour change. As in the case 

 of Bothtis, the time required by a particular individual to copy the ground could be 

 decreased by repetition. 



Recently, Hewer (1926, 1931) has analy.sed the colour patterns of Psettodes and a 

 number of British Flatfishes. He remarks that the permanent markings " exist for 

 the most part as morphological entities, and that the background on which the fish 

 is placed induces the nearest approximation for the fish concerned ". The distribution 

 of these .specialised spots, such as the dark patches, orange and black, and white 

 spots, is regarded as being of an ancient nature, such a distribution being fairly constant 

 in Psettodes, and also being found in some form or another in all the other species 

 examined. The dark patches found in so many Flatfishes, of which the two situated 

 on the lateral line are perhaps the most characteristic, are believed by this author 

 * See also Cunningham and McMunn (1891). 



