Chromatophores and Color Change 711 



ically the older, and that direct nervous control has been superimposed upon 

 it in those fishes of more recent evolutionary origin.^"** The latter tendency 

 is associated with more rapid response to background, which has become 

 possible through a simultaneous increase in the speed of the melanophore 

 change itself. The typical teleost controlling mechanism for melanophores is 

 diagrammed in Figure 273. 



A survey of the character of the response of melanophores of fishes to 

 war gases suggests the presence of two differently responding types, with 

 catfishes possessing one type and scaly fishes the other. ^^''- 



Very much less is known about the control of the erythrophores, xantho- 

 phores, leucophores, and iridocytes than about the control of the melano- 

 phores. The erythrophores of the squirrel-fish, Holocentrus, are rapidly re- 

 sponding effector organs.^'^- Through their activity the fish can change from 

 red to white in about 5 seconds, and make the reverse change in about 20 sec- 

 onds. These responses may be induced by change from black to white back- 

 ground, and vice versa. Transection of nerve tracts in a fish on a white back- 

 ground results in dispersion of red pigment. The areas blanch again in a short 

 while and may be darkened again by a second more distal cut, indicating the 

 presence of dispersing nerve fibers. The presence of concentrating nerve fibers 

 can be demonstrated by electrical stimulation of the medulla, resulting in 

 rapid paling of all innervated erythrophores. Experimentally denervated cells 

 fail to give this response. Adrenalin concentrates the pigment. Pituitary ex- 

 tracts from other squirrel-fish produce no effect when injected into normal 

 light-adapted or dark-adapted specimens. It thus appears that the erythro- 

 phores of Holocentrus normally are exclusively under nervous control.^^^ 

 The erythrophores of Phoxinus, on the other hand, have been shown to be 

 influenced by a principle from the hypophysis.^" 



The xanthophores of Funduliis appear to possess double innervation, 

 comprising concentrating and dispersing fibers.^^ A concentrating hormone, 

 probably adrenalin, also appears to be responsible for the concentration of 

 the pigment which results from handling of the fish. This latter concentra- 

 tion occurs as rapidly in denervated xanthophores as in innervated ones. 

 On the other hand, intraperitoneal implantation of Fundulus pituitaries into 

 hypophvsectomized specimens induces pigment dispersion in denervated xan- 

 thophores, regardless of color of background. Such implants also impede the 

 typical pigment-concentration in innervated cells in response to blue or 

 white backgrounds."^ It therefore appears that several factors normally in- 

 Huence the state of the xanthophores in this species. Melanophores and xan- 

 thophores of Fundulus react independently in background responses.^ 



A number of fishes, including Fundidus, possess non-iridescent reflecting- 

 white chromatophores known as guanophores or leucophores.'*^ These may 

 show physiological changes in the adaptive responses of the fish to back- 

 ground. Little is known of their normal control. They continue to respond 

 to background after hypophysectomy, and after sufficient dosage of the fish 

 with ergotamine to prevent any response of the accompanying melanophores. 

 They disperse their pigment under the influence of adrenalin. ^-^ 



Iridosomes or chromatosomes play only a passive role in adaptive color 

 changes, becoming more or less obscured through activity of the other chro- 

 matophore types. Normally these bodies are green or blue. They are highly 



