IX THE SKIN 197 



Hyla placed where the skin comes m contact with rough 

 substances become very dark in color even when surrounded 

 with bright-colored materials. Hylas which were placed 

 upon smooth green leaves became light colored even in the 

 dark. While the influence of light is admitted to be an 

 important factor, the color changes of Hyla are regarded by 

 Biedermann as determined to a great extent by the nature 

 of the material with which the skin comes in contact. Since 

 in the life of the tree frog rough surfaces are generally asso- 

 ciated with a dark environment, while smooth surfaces are 

 usually afforded by green leaves, this method of reaction to 

 contact" stimuli conspires to bring about, in most cases, an 

 adaptation of the color of the animal to that of its sur- 

 roundings. In the species of Rana studied this mode of 

 reaction to contact was not observed. Finally it may be 

 observed that color changes are associated with the psychic 

 states of the animal. Frogs, like men, may turn pale through 

 fear, but the mechanism of the process is very different in 

 the two cases. If frogs are held in the hand for some time, 

 the skin turns paler ; this may in part be a reaction to tem- 

 perature, but the same effect is produced if the animal is 

 pursued and caused to jump about vigorously in its attempts 

 to escape. 



Absorption and Excretion. — The power of the frog's skin 

 to absorb water has already been described. The skin does 

 not function in absorption like a dead membrane. The 

 facility with which fluids pass through the skin from without 

 inwards is quite different from that with which they pass in 

 the reverse direction. According to Reid a five per cent 

 sugar solution in distilled water passes through the skin more 

 rapidly from within outward than from without inward ; but 

 if the same percentage of sugar is dissolved in a normal 

 salt solution, the fluid will pass more rapidly from without 



