286 THE VERTEBRATE ANIMALS 



Locomotion. You will notice that the appendages have the 

 same general position on the body and same number of parts as do 

 your own (upper arm, forearm, and hand ; thigh, shank, and foot, 

 the latter much longer relatively than your own). Note that while 

 the hand has four fingers, the foot has five toes, the latter connected 

 by a web. In swimming the frog uses the stroke we all aim to 

 make when we are learning to swim. Most of the energy is liber- 

 ated 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, called the nictitating mem- 

 brane (or third eyelid), 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. 

 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 becomes colder than that of the 

 water. 



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, thus 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 respiration. 



Although we shall take up the study of the internal structure of 



