LIGHT AND PIGMENTATION 



85 



The mechanism oj the colour change varies in diilerent sjDecies. In 

 the simplest unicellular form, Euglena, a red hsematochrome pigment 

 migrates from a deeper position beneath the green chloroj)lasts to 

 disperse itself superficially under the influence of light (Johnson. 

 1939) ; but the most common mechanism is through the activity of 

 special integumentary cells ^ called chromatophores (xptD/Lia. colour ; 

 (f)6pog, a carrier). Occa.sionally the phenomenon is morphoJogicaJ 

 involving a change in the number of functioning chromatophores or an 

 alteration in the quantity of pigment in each. This, a relatively slow 

 mechanism, is well exemplified in the pigmentation that can be 

 induced in the white belly of flounders if normal fish are kept in a black 

 tank or are blinded and are illuminated from below (Osborn, 1940). 

 Sometimes, as in certain insect larvae and spiders, this is the sole 

 mechanism of colour-change available (Gabritschevsky, 1927) ; it is 

 usually less obvious and impressive than the more common method 

 which is responsible for the dramatically vivid colour changes in 

 poikilochromic Crustaceans, Fishes, Reptiles and Amphibians. ^ This — 

 a functional or j^hysiological change — involves merely a redistribution 

 of pigment. The change may be effected by a single pigment which at 

 one time is concentrated into tiny spots lost in a pallid background of 

 skin so that it contributes little to the colour of the animal, at another, 

 dispersed so that the animal becomes apj^ropriately tinted. Alterna- 

 tively a pigment of one colour may stream in front of or retire behind 

 pigments of other colours so that surprising changes of hue may raj)idly 

 occur. The two methods, morphological or ph3'siological, are not 

 mutually exclusive, for if the conditions determining the second are 

 maintained for a sufticiently long time, permanent morphological 

 changes tend to occur, a generalization sometimes known as Babak's 

 law (1913). 



Warm-blooded animals, on the other hand, can only change their colour 

 by the slow and laborious process of renewing their inert feathers or hairs ; to 

 this there are a few exceptions wherein specific stratagems are adopted, .such as 

 the antelope-jackrabbit which turns white in its flight by rolling up the skin of 

 its belly on the side towards its pursuer. 



Two major tyj^es of chromatophores occur. In Molluscs (cuttle- 

 fish and their allies, squid, octojDus), the chromatophores are in highly 

 organized groups of cells in which the pigment is redistributed by 

 neuromuscular activity. Each organ consists of a central cell filled 

 with pigment (red. brown or yellow) around which radiate a number of 

 muscle-fibres which, on their simultaneous contraction, pull out the 



r 



^ In certain transparent Fishes part or al) of the colour pattern is found in interna] 

 organs such as the peritoneum and meninges. 



^ Crustaceans, Keeble and Gamble (1903-5) ; various Vertebrates, Babak (1913), 

 Brown (1934) ; Fishes, Odiorne (1933) ; Amphibians, Sumner (1935). 



