COLOR CHANGES IN ANIMALS — CARLISLE 



421 



Figure 2. — A chromatophore of a squid: left, contracted, and, right, expanded. Expan- 

 sion is brought about by contraction of muscles which spread out the sac of pigment 

 to make a thin broad plate. 



with it concentrated, the tiny dots are almost unnoticeable and the skin 

 is blanched. 



This type of chromatophore is widely distributed throughout the 

 animal kingdom. It is found in fish, amphibians and reptiles, prawns 

 and shrimps, sea urchins and insects. Another type of chromatophore 

 is confined to the cephalopods, that group of mollusks which comprises 

 the squid, cuttlefish, and octopus. Here the pigment — mainly various 

 shades of melanin from the lightest of browns to jet black — is con- 

 tained within spherical sacs each surrounded by a corona of muscle 

 fibers (fig. 2). When these are relaxed the chromatophore is con- 

 tracted to an insignificant sphere, but when the muscles contract the 

 sac is drawn out into a tliin plate covering some hundred or more 

 times the area. 



FROM STIMULUS TO RESPONSE 



The final topic I propose to consider is the question of the mediation 

 between stimulus and response. What means does the animal adopt to 

 pass on the stimulus received by the senses to the chromatophores which 

 produce the color change. In the simplest situation the effector organ 

 responds directly to the stimulus — the human skin produces melanin 

 as a direct response of each individual chromatophore to the ultra- 

 violet radiation. A single chromatophore of a prawn may react to 

 a spot of light, and a single chromorrhiza of a sea urchin has been 

 found to react to a beam of light a ten-thousandth of an inch in diam- 

 eter. The cells of the feather follicles of flamingos lay down the pink 

 carotenoid in the feather they are forming in direct ratio to the 

 amount supplied by the blood. In these examples of color change — 

 both morphogenetic and kinetic — no central control is involved ; the 

 stimulus is received and the response is carried out by the effector 

 cell itself. 



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