172 ANIMAL BIOLOGY 



is only moving objects that attract frogs or toads, for they 

 will go hungry in the midst of plenty unless some motion 

 induces them to respond. 



Frogs never drink as higher animals do but they ob- 

 tain water .by absorption through the skin. They lose 

 water very rapidly by evaporation when kept in a dry 

 atmosphere and present a much shrivelled appearance, 

 but if placed in water again they soon become plump. 

 Dryness is soon fatal to frogs and toads and they conse- 

 quently rarely venture far from water, or at least they 

 seek a moist retreat. The skin of these animals is an im- 

 portant organ of respiration as well as of absorption. 

 While under ordinary circumstances respiration is carried 

 on by both lungs and skin, skin respiration alone may suffice 

 to maintain life if the animal is at a low temperature. 

 Frogs often bury themselves in the mud at the bottom of 

 ponds during the winter when respiration is naturally 

 carried on through the skin alone. 



Frogs (Ranidae) are found in nearly all countries 

 of the globe except in the colder latitudes where they can- 

 not escape being frozen in the winter. The most common 

 of our many North American species is the leopard frog, 

 Rana pipiens, which ranges over a large part of the middle 

 and eastern sections of the United States. Our largest 

 species is the bull-frog, Rana catesbiana whose very hoarse 

 croak resembles the roaring of a bull. It is generally 

 found in or near water and has been sought so much for 

 food that it has been very much reduced in numbers and 

 is practically extinct in many regions where it was once 

 abundant. 



Toads (Bufonidae) are generally more terrestrial in 

 habit than frogs. The rough warty appearance of their 

 skin is due to the development of large poison glands 

 whose secretion is quite irritating to sensitive surfaces. 



