76 BRAIN AND BODY OF FISH 



for regarding this fish as the least speciaUsed member of the flat- 

 fishes (Heterosomata), and to have Percoid affinities. The usual 

 explanation of the origin of flat-fish is that at some time in the distant 

 past their ancestors acquired the habit of resting on their side on 

 the bottom, and even sleeping in that position. It has been observed 

 that Wrasses still have that habit. The view is held that some 

 particularly lazy or drowsy fish adopted this attitude as a pleasant 

 way of passing the time, and incidentally discovered that by lying 

 on the bottom it was less molested by enemies. But the chief 

 effect that resulted, was that either the sand was apt to get into the 

 bottom eye, or that a permanent squint developed. 



However, the process of adaptation caused the bottom eye to 

 migrate to a dorsal position by passing through the front of the head 

 in the manner we have described above ; another effect of this 

 recumbent position was the loss of pigment on the bottom side, 

 which is apparently due to the absence of light, as young flounders 

 kept in a tank with a glass bottom on which light was projected, 

 in the course of time, have been found to form pigment on the white 

 side. Flat-fishes have a remarkable power of making the skin 

 pattern on their dorsal side conform to their surroundings and as 

 vision was necessary to review the background against which they 

 lay, a further adaptation occurred. A sac-like profusion of the 

 membranous wall of the orbit developed, which like the cavity of 

 the orbit is filJecl Avith fluid. This enables the eye to be protruded 

 above the surface of the head to a great extent, and aUows free 

 movement. It is signiflcant that this protrusion is very marked in 

 the plaice, a fish richly endowed with pigment cells, some of a 

 briUiant colour, and that the optic lobes are remarkably A\eU- 

 developed and almost bi-lobed in shape. 



Flat-fishes or Heterosomata have recently been the subject of 

 a valuable monograph by Norman. In it he discusses the different 

 views as to the origin of flat-fishes, whether they have been derived 

 from a single stock of either the cod type, dory type, or perch type, 

 or from a number of stocks. Norman regards the theory that they 

 have arisen from a generalised percoid stock as the most probable. 

 He considers Psettodes to be the least specialised member of the 

 order, and that the soles and Cynoglossidae may have arisen from 

 another part of the percoid stem. After studying this monograph, 

 the last volume of the Journal of the Marine Biological Association 

 reached my hands, and it was with some interest that in an article 

 by Ford on the vertebrae of Teleostean fishes, the similarity of the 

 backbone of Psettodes with that of the generalised percoid type was 

 pointed out. 



