156 BRAIN AND BODY OF FISH 



The flat fishes have adopted a recumbent posture wliich gives 

 them protection from enemies and conceahnent from their prey ; 

 at the same time this protection is dependent on adaptive coloration 

 -and conformity to their background. To enable the fish to do this, 

 the background must be surveyed and, by some unfathomed optical 

 reflex, the fish becomes more skilled than the leopard, and changes 

 its spots, and thus produces the desired design. The great develop- 

 ment of the eyes and optic lobes in the plaice have been 

 described, but the higher centres, which presumably carry out the 

 artistic effects, are unknown ; but there must be some central 

 nervous mechanism present to control the nerve plexuses that 

 surround each pigment cell in the skin. The reflex artistry of the 

 plaice, which with natural pigments reproduces on its own canvas 

 a picture of its background, by what must, under the best of 

 circumstances, be an oblique view is one of the astounding pheno- 

 mena of nature. 



The observations on the protrusion of the eyes and the recessus 

 orbitahs are most remarkable. The migration of the eye, and the 

 anatomical details that accompany this change of outlook, have been 

 described ; but a further adaptation became necessary after the 

 eye had reached its comparatively exalted position, as the range 

 of vision was not sufficiently extensive to obtain a view of the 

 surrounding ground ; this apparently was the case with the upper 

 eye. It was necessary that some mechanism should be evolved 

 to allow the eye to be protruded ; so it appears that a hydi'aulic 

 sac was develojied in connection with the membranous wall of the 

 orbit, and this allowed the eyeball to be protruded to a remarkable 

 extent. Now the protective coloration ensures the survival of the 

 fish, but before this protection could be obtained an extensive view 

 of the surroundings was essential and this was not possible until 

 this mechanism had been evolved ; therefore the recessus orbitalis 

 must have been formed before any protective colour change could 

 have become efficient. It would seem that the greater the power 

 of protrusion of the eye, the more efficient is the power of colour 

 adaptation, as is exemplified by a study of the plaice. 



Therefore the history of the development of a flat-fish recounts 

 not only the adoption of the recumbent lateral posture, the migration 

 of the under eye to an upward site, and the loss of pigment on the 

 under side of the body, but the development of a high j)ower pro- 

 jecting optical apparatus with reflex colour-photographic control 

 of the chromatophores. An interesting fact has also been established 

 in regard to colour changes, that the time required by a particular 

 individual to copy the ground could be decreased by repetition, 



