NEUROHUMORS 55 



cerned the pituitary secretion differs from that in the 

 fin only in the much greater distance that the former 

 must cover (Parker, 1934^). As previously suggested, 

 all these agents, be they short-range or long-range, acti- 

 vate the melanophores in essentially the same way. 

 Hormonal excitation and nervous excitation so far as 

 color-cells are concerned are really one in principle; both 

 are carried on by special hormones, the activating neu- 

 ron u mors. 



It would be quite impossible at present to attempt a 

 catalogue of neurohumors. From what has been men- 

 tioned in discussing the conditions in Fundulus and in 

 Mustelus there appear to be at least two classes of these 

 substances, the water-soluble and the oil-soluble or, as 

 they have been designated, hydrohumors and lipohu- 

 mors (Parker, 1935^). Hydrohumors are soluble in 

 water and especially in blood, lymph, or other watery 

 body fluids. In consequence they spread rapidly and 

 far and ordinarily bring about responses over the whole 

 animal. They are well represented by the chromato- 

 phoral secretions of the pituitary gland as seen in Mus- 

 telus^ Ameiurus and a host of other creatures. Lipo- 

 humors are soluble in lipoids, fats, fat solvents and the 

 like. Since such substances are essentially stationary 

 in the animal body, the lipohumors after dissolving in 

 them must diffuse through them and consequently move 

 very slowly from place to place. Lipohumors are there- 

 fore relatively local in their action and do not excite 

 responses of the body as a whole. They are appropriate 

 to animals that can change their color patterns and 

 maintain them thus changed as, for instance, certain 

 flatfishes. When such fishes are on a coarsely varie- 

 gated background they show an appropriately coarse 

 melanophore pattern which is strikingly readjusted to 



