Introduction 



tion, been heard to produce sounds. Further investigation will 

 probably prove all to have the power of producing weak sounds 

 of some sort. Many frogs and toads, both male and female, 

 give a high-pitched cry when greatly annoyed, and some of the 

 frogs open the mouth and produce a scream so loud and so much 

 like the human voice that it is startling in effect. The female 

 bullfrogs (Rana catesbiand) of Wisconsin can give the low-pitched 

 "jug-o-rum" call with a vigor almost equal to that of the male, 

 also swelling the internal vocal sacs to a size almost equal to 

 those of the male. 



X. Colour and Colour Change 



Colour in the Salientia is largely due to pigment in the skin. 

 This pigment may be black, red, yellow, or metallic. The black 

 pigment is granular and is enclosed within cells (chromatophores) 

 which have the power of changing shape. With a low power of 

 the microscope, these dark, branched cells can easily be seen in 

 the thin web of a frog's foot or in the fin of a tadpole's tail. There 



^— is usually a layer 



^r of black pigment 



^^ cells just below 



^ ^ the epidermis, 



^ ^_ ▼ which, itself, is 



^H ^ white and trans- 



^ w parent. These 



^ ^ V^ pigment cells 



& Ate ^^^ contract or 



^ ^ A ^"^ expand, radiat- 



(^ H^ ingmanybranch- 



^ ^ es. If they con- 



^ 1M w tract and retreat, 



-| the surface of 



^ the frog is left 



^ W light in colour 



Fig. lo. When the pigment cells retreat and contract, ^ j . 



the resulting color of the skin is light. expand, Stretch- 



ing OUt black an- 

 astomosing branches toward the transparent epidermis, the skin 

 appears dark (Fig. 1 1). Each cell is connected by means of a 

 slender nerve fibre with the sympathetic nervous system, which 



22 



