COLOR CHANGES IN ANIMALS — CARLISLE 419 



to breed. Mention of the mallard makes it necessary to enlarge 

 the scope of the definition of morphogenetic color change, for the 

 iridescent head of the breeding mallard owes its sheen not to pig- 

 ment but to the structure of the feathers which produces color by 

 purely physical means. Thus morphogenetic color change may m- 

 volve the shedding or acquiring of structures which owe their color 

 to optical interference, as well as the loss or gain of pigment. 



Kinetic color changes too may occur as responses to a wide range 

 of stimuli. Many of the shrimplike creatures of the plankton — the 

 copepods and others — respond to unwelcome actinic radiation by 

 expansion of pigment to form a sort of umbrella over the sensitive 

 organs, just as we respond to similar unwelcome radiation by tan- 

 nmg. Prawns (shrimps) can change color to match the backgromid — 

 and pattern too, as well as color — within about a quarter of an hour, 

 and octopus and squid can complete within a second a change that con- 

 ceals them. The three-spined stickleback, the cuckoo wrasse, and the 

 black sea-bream can adopt a breeding dress quite as colorful as that 

 of the drake mallard, but by kinetic means. High temperatures and 

 bright light can stimulate a prawn to change color and attack by a 

 predator can stimulate a cuttlefish to produce the "eyespots" which 

 serve to frighten off the larger animal. Even the chameleon, which 

 changes color so slowly, shows more than a mere background or albedo 

 response, for its response to background color is modified by the 

 temperature. 



EFFECTOR ORGANS 



The stimuli which provoke color change are thus many and vari- 

 ous, but the effector organs themselves which bring about the changes 

 are more uniform. The pigment is largely contained within cells, 

 usually in droplet or granular form, rarely dissolved. The cells con- 

 taining the pigment may be dead — as in feathers, which are largely 

 composed of dead cells — and then color change can only take place by 

 morphogenetic means. Sometimes the pigment is found in the gen- 

 eral cells of the skin, as in the stick insect. In this animal all the 

 cells of the skin contam at least two kinds of pigment. About half- 

 way down the cell is a fixed barrier of light colored pigment gran- 

 ules. Granules of a black pigment are free to migrate up and down 

 the cell turning the animal black when they lie at the surface or pale 

 when they are hiding behind the barrier of light pigment. More 

 commonly the pigment is confined to special cells known as chroma- 

 tophores. Even in dead strictures such as hairs the pigment is not 

 uniformly distributed but confined to once living but now dead 

 chromatophores. It should be obvious that where the greater part 

 of the body is covered by such dead structures any color change can 

 come about only by shedding these structures and growing fresh ones 



