Chromatophores and Color Change 



707 



mary responses but to have initially the secondary phase. Species of the 

 latter type include Fiindulus,^^- -^^ Lehistes, Xipho-phoriis, Gamhusia,^^^ 

 Scyllium,^'-^'^ and Mustelus.^-^ A number of fishes normally showing second- 

 ary color responses will revert to primary responses after blinding. ^'^- 



The secondary color responses of fishes are dependent typically upon the 

 eyes. They involve nervous pathways to the central nervous system, thence 

 either to endocrine glands affecting the chromatophores through blood-borne 

 hormones, or by way of efferent nervous pathways directly to the chromato- 

 phores where chemical mediators are liberated by the nerve terminations. 

 Both hormonal and nervous mechanisms may cooperate in many cases. There 

 is considerable variation among fishes as to the normal mechanism of con- 

 trol. Parker,^'^*^ the leading investigator in the field of animal color changes, 

 divides the fishes into three groups on the basis of the degree to which di- 

 rect innervation of the melanophores is found. Dineuronic chromatophores 

 possess double innervation with separate dispersing and concentrating fibers. 

 Mononeuronic chromatophores possess single innervation in which the ac- 

 tivity is always pigment-concentrating. Aneuronic chromatophores possess 

 no innervation, their responses being due solely to activity of blood- and 

 lymph-borne chemical factors. 



Fig. 272. A, Dark band in caudal fin of white-adapted Fundulus produced by severance 

 of radial nerves. B, Redarkening of a faded band following a second, more distal cut. 

 From Parker.^ 



The great majority of the teleost fishes thus far carefully investigated ap- 

 pear to possess dineuronic melanophores. One type of innervating nerve 

 fiber, a pigment-concentrating one, is readily demonstrated by electrical 

 stimulation of appropriate loci in the central nervous system, of central nerve 

 tracts, or of peripheral nerves. The extent of the area of the skin blanching 

 under this stimulation parallels the area of the skin innervated by these 

 nervous elements. Furthermore, after denervation of any area of the skin the 

 denervated area no longer exhibits blanching responses to stimulation of the 

 nerves central to the point of denervation, even though responses may con- 

 tinue in the adjacent areas. Therefore the blanching produced by nerve stim- 

 ulation in the animals is a result of localized responses at the region of the 

 ner\'e terminations and is not due to freely diffusible substances in the 

 blood. The action of the concentrating fibers is believed by Parker^^^ to be 

 mediated through adrenalin-like material which is liberated by the nerve 

 terminations and which diffuses through the tissues. 



