^g2 Comparative Animal Physiology 



phore processes to a position superficial to the inert light-colored layer, there- 

 by concealing the latter''''- '^^ (Fig. 257). 



The rate ol physiological color change is limited by the rates of mechanical 

 response of the effector organ and of its controlling mechanism. There is, 

 however, great variation in such rates among animals. The squid is able to 

 carry out maximum color change in a matter of seconds, as is also the squir- 

 rel-fish, Holocentrns. A few minutes suffice for maximum color change in 

 the minnows, Fiindtiliis and Lehistes. From one to several hours are needed 

 by many crustaceans, insects, and the catfish, Ameiurns, and days are re- 

 quired for comparable maximum changes in the flatfishes, the eel, Anguillula, 

 elasmobranchs, and amphibians. 



A number of methods have been utilized in the measurements of physio- 

 logical color changes. These have been critically reviewed by Parker. ^^'* None 

 of the methods permit complete differentiation between influences which 

 are in part the result of the morphological color changes and those which 

 are purely physiological. Instead, the measurements are generally based on 





^ -^m' '!^^w ''^-'W^^^ 



I 2 30 4 50 



Fig. 258. Melanophores showing various degrees of pigment dispersion. 1, Punctate; 2, 

 punctostellate; 3, stellate; 4, reticulostellate; 5, reticulate. From Hogben and Slotoe." 



time intervals of sufficient brevity to assure that morphological changes 

 would not have influenced the results significantly. One group of methods 

 employs simply the gross changes in color of the animal as an index of the 

 extent of dispersion of the dark pigments. This may involve a visual deter 

 mination in which the animal is merely described as being light, dark, or 

 intermediate, or in which subjective grades of variation between known 

 extremes are estimated and expressed numericallv"*^ in four or five grades. 

 Some of the subjective aspects have been removed by a method employing 

 photometric determination of the fraction of the incident light reflected from 

 a unit area of skin surface, "'* or the relative amounts of light transmitted by 

 isolated fish scales. ^''•^ 



A second group of methods has been based, not on the gross light ab 

 sorptive changes in the skin, but rather on the changes in the chromato- 

 phorcs themselves. One of these methods was measurement of the actual 

 diameter changes in the individual chromatophores. This method was em- 

 ployed originally by Spaeth"^' for fish melanophores and has since also been 

 applied to crustaceans.--^ Another method of this type, and one rather exten- 

 sively adopted, is one originally proposed by Slome and Hogben'^- (Fig. 

 258), in which the chromatophore state was numerically described as fol- 

 lows: 



1 = Punctate; 2 = Punctostellate; 3 = Stellate; 4 = Reticulo-stellate; 

 5 = Reticulate. This system has the advantage that quick inspection can, 

 after a little practice, yield a numerical value which is adequate for many 

 comparative purposes. 



