242 



A STUDY OF VERTEBRATES 



all aim to make when we are learning to swim. Most of the energy 

 is liberated from the powerful backward push of the hind legs, 

 which in a resting position are held doubled up close to the body. 

 On land, locomotion may be by hopping or crawling. 



Sense Organs. The frog is well provided with sense organs. 

 The eyes are large, globular, and placed at the side of the head. 

 When they are closed, a delicate fold, or third *'eyelid, called the 

 nictitating membrane, is drawn over each eye. Frogs probably 

 see best moving objects at a few feet from them. Their vision is 

 much keener than that of the fish. The external ear (tympanum) 

 is located just behind the eye on the side of the body. Frogs hear 

 sounds and distinguish various calls of their own kind, as is proved 

 by the fact that frogs recognize the warning notes of their mates 

 when any one is approaching. The inner ear 

 also has to do with balancing the body as it has 

 in fishes and other vertebrates. Taste and smell 

 are probably not strong sensations in a frog or 

 toad. They bite at moving objects of almost 

 any kind when hungry. TJie long flexible 

 tongue, which is fastened at the front, is used to 

 catch insects. Experience has taught these 

 animals that moving things, insects, worms, and 

 the like, make good food. These they swallow 

 whole, the tiny teeth being used to hold the 

 food. Touch is a well-developed sense. They 

 also respond to changes in temperature under 

 water, remaining there in a dormant state for 

 ,, . the winter when the temperature of the air 



This diagram snows *" 



how the frog uses becomes colder than that of the water. 



its tongue to catch 

 insects. 



Breathing. The frog breathes by raising 

 and lowering the floor of the mouth, pulling 

 in air through the two nostril holes. Then the little flaps over 

 the holes are closed, and the frog swallows this air, forcing it 

 down into the baglike lungs. The skin is provided with many 

 tiny blood vessels, and in winter, while the frogs are dormant 

 at the bottom of the ponds, it serves as the only organ of respi- 

 ration. 



