1 86 Descriptive Zoology. 



the two eyes. In some kinds of frogs the head is more 

 distinctly green than other parts, so a little practice is 

 required before the collector readily discovers it as it rests 

 in the water, with the top of the head appearing among 

 green leaves. 



The color is due to pigment cells in the deep layer of the 

 skin. These cells are branched, but can change their shape 

 and vary the color somewhat in accordance with the sur- 

 roundings. These cells may easily be observed in the skin 

 of the frog's web, and can hardly escape observation when 

 the circulation in the web is studied. 



The Frog's Enemies. Most of the larger snakes eat frogs 

 when they can get them. Many fishes take them greedily, 

 hence the frog is used as bait. A number of birds capture 

 frogs, including some of the hawks, certain waders, and 

 perhaps others. 



The frog's color undoubtedly often keeps it from being 

 discovered, and when it is approached it can make use of 

 one, or both, of its two speedy modes of locomotion to make 

 good its escape ; concealment by protective resemblance 

 and escape by flight are its two safeguards, for it has no 

 weapons of defense. 



How Frogs spend the Winter. At the approach of freez- 

 ing weather frogs reassemble at the shore, and perhaps for 

 some time may be found at the surface during the warmer 

 part of the day, but stay at the bottom during the night or 

 during colder days. They finally dive deep into the mud, 

 which is their winter resort. Here they hibernate, motion- 

 less, eyes closed, lungs emptied, with no breathing move- 

 ments, the heart beating feebly and slowly, till they are 

 revived by the returning warmth of spring. 



Warm-blooded vs. Cold-blooded Animals. The frog's nor- 

 mal temperature during the season when it is active is a 



