CLASS AMPHIBIA 



523 



Color changes are brought about by direct stimulation of the 

 pigment cells or indirectly through the central nervous system. 

 Light is an important stimulus; it acts both directly and 

 through the central nervous system. In a bright light the skin 

 of the frog becomes light in color, whereas in the dark it changes 

 to a darker hue. Temperature i another important factor. 

 The pigment becomes more concentrated if the temperature is 

 raised, and the skin changes to a lighter color. An expansion 



FIG. 437. Pigment cells from the frog, in different states of extension. 

 (From Holmes, after Verworn.) 



of the pigment and a darker color result from subjection to cold. 

 Changes in the circulation, in the moisture of the frog's habitat, 

 and in the chemical composition of the animal's environment affect 

 the chromatophores and consequently produce changes in color. 

 In many cases the color changes are such as to cause the frog 

 to resemble more closely its surroundings, and hence to conceal it. 

 Regeneration. The power of regenerating lost parts is 

 remarkably well developed in many AMPHIBIA. For example, 

 the hand of a two-year-old axolotl was cut off, and in twelve weeks 



