XXIV TEGUMENTAEY SYSTEM. 



coloured, may be present and capable of changing their form under special 

 influences, which are apparently directly connected with impressions of 

 colour received by the eye and brought about by the reflex action of the 

 nervous system. Under the heading of Pleuronectida3 (vol. ii, p. 1) remarks 

 will be found how these flat-fishes rapidly change their colour on differently 

 coloured bottoms, thus obtaining the protection of concealment by adaptation 

 of colour. 



The chromatophores or pigment cells are seen in the early stages of the 

 eggs of fishes, but as observed by Agassiz, in some we have even two colour 

 elements in the older stages, immediately before the young fish is hatched, 

 viz. the black and yellow ; still in the majority of cases the black alone is 

 present, the yellow element appearing subsequently, and last of all the red. 

 Pouchet considered the blue pigment merely a dimorphic condition of the 

 red pigment ; the same may also be said of the green. And with the 

 growth of the fish the capacity of the chromatophores for expansion rapidly 

 augments. 



In addition to the chromatophores another set of bodies termed 

 iridocytes, and more or less analogous to excessively thin laniiuas, have been 

 observed situated near the surface. By simple combinations of the action of 

 the red, yellow, and black chromatophores with the iridocytes are obtainable 

 all the colours producible in fish. These result mainly from expansion 

 near the surface, or retraction into an inferior layer of the chromatophores, 

 which thus mixed with the yellow and red, or with the iridocytes at greater 

 or less depth suffice to produce the variations of colour. 



These pigment cells are likewise said to involuntarily expand owing to 

 external irritation, as from the muscles of a fish convulsively contracting a 

 short time prior to death, thus accounting for the rapidly changing tints in 

 some which are shown subsequent to the period of capture ; and the satiny- 

 red skin of the red mullet is in some places thus produced by fishermen 

 who scale their fish soon after their being captured. 



Its appears evident that the influence of light is mostly felt through the 

 eye. Pouchet found- that turbots if blinded did not change colour, but those 

 not deprived of sight did: and young -hybrid salmonidse raised at Howietoun 

 .in which vision was more or less deficient, were observed to be generally lighter 

 in colour than their fellows, and their fins to become red as they grew older. 

 Whether the degeneration of the eye perceived in cave fishes and those of 

 the deep sea is due to dimness of this organ or an effect of hereditary trans- 

 mission has yet to be shown. A not infrequent change in some fresh-water 

 forms is a yellow colour taking the place of the original tint : thus the 

 normally dull greenish tench may be seen of a brilliant orange yellow, 

 termed leuccethiopism or' xanthochroism. So may likewise the gold carp, 

 although this latter when in a wild state in China is' of a dull green. The 



