710 



Comjparative Animal Physiology 



of both concentrating and dispersing nerve fibers, color changes seem pre- 

 dominantly determined by hormones. In its activity in the eel, the B-sub- 

 stance is believed to be assisted by a W-substance.^^^ That direct innervation 

 does play some role in color changes in this fish is seen in the limited back- 

 ground response after hypophysectomy. 



The B-substance is found to be slighdy less important in normal color 

 changes in the catfish Ameiurus. Hypophysectomized catfishes continue to 

 show color changes in response to black and white backgrounds but show 

 only an intermediate degree of darkening on black.-'^ Injection of posterior- 

 lobe extract will, however, completely blacken these fish. Here we must 

 assume that a blood-borne B-substance supplements the action of dispers- 

 ing nerve fibers in the normal responses to black backgrounds; there is as 

 yet no evidence for the operation of a W-substance in this species. 



The killifish, Fundulus, on which a vast amount of research has been 

 done, is a species in which the dominant mechanism of melanophore control 

 is nervous. Color changes are very rapid, only a minute or two being re- 

 quired for nearly maximal color change. These changes continue to occur 



W(B) 



Fig. 273. Diagram of the controlling mechanism of melanophores in the eel, Anguilla. 

 L, incident light; W(B), white or black background; DR, dorsal retina; VR, ventral 

 retina; CNO, central nervous system; P, pituitary; A, adrenergic fibers; C, cholinergic 

 fibers; B, blood and Ivmph; DM, dispersed melanin; CM, concentrated melanin. From 

 Parker.^=^ 



in hypophysectomized specimens.^^^ Furthermore, injection of extracts of 

 posterior lobe into pale fish on a white background does not produce sig- 

 nificant darkening. Since such extracts will induce darkening in denervated 

 areas of the skin we must conclude that the chromatophores are normally 

 influenced to some extent by this substance, which is shown to be present 

 in their pituitaries."- ^'■^ However, its normal influence is probably seen only 

 in the production of extreme conditions of dark adaptation maintained over 

 relatively long periods of time. 



All teleost fishes thus far investigated appear to fall into a series in the 

 relative influences of direct innervation and blood-borne hormones. A reason- 

 able hypothesis has been advanced that the humoral control is phylogenet- 



