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ANIMALS AS LIVING ORGANISMS 



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

 The e3^es are large, globular, and placed at the sides of the head. 

 Sometimes you can see a delicate fold, or third eyelid, called the 

 nic'titating membrane, drawn over each eye. A frog's vision is 

 much keener than that of a fish ; it is probably best for moving 

 objects at the distance of a few feet. The external ear {tym'- 

 panum) 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 in- 

 ner ear has to do with balancing the body, 

 as it has in fishes and other vertebrates. Taste 

 and smell are probably not strong sensations 

 in frogs or toads. They bite at moving objects 

 of almost any kind when hungry. The 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. 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. 

 Organs and Functions of the Higher Animals. — The same 

 general functions performed by a single cell are performed by a 

 many-celled animal. But in the many-celled animals the various 

 functions of the single cell are taken up by the organs. In such 

 a complex organism as the frog or man, the organs and the func- 

 tions they perform may be briefly given as follows : — 



(1) The organs of protection, such as the skin and the outside 

 skeleton seen in a turtle. 



(2) The organs of /oo(i ^a/cmg.' the mouth and parts which place 

 food in the mouth. 



(3) The organs of digestion : the food tube and the collections of 

 cells which form the glands connected with it. The enzymes in 



This diagram shows 

 how the frog uses its 

 tongue to catch in- 

 sects. 



