AN INTRODUCTION TO ANIMAL BIOLOGY 27 



to have radial symmetry. There are comparatively few 

 such animals, but numerous plants are so arranged. 



The word organ will be used frequently hereafter. An 

 organ is a part of the body of a plant or animal fitted 

 for doing a particular work. For example, the heart is an 

 organ for pumping blood, lungs are organs for breathing, 

 muscles are organs for moving. A group of organs doing 

 similar work form a system. Thus all the muscles constitute 

 the muscular system, which is a group of organs adapted to 

 the work of producing movement. 



At this point it will be well to have at hand a dead frog, one 

 either recently chloroformed or preserved for some time in formalin. 

 Examine the dead frog for all points not easily seen in the living 

 one kept in the glass tumbler. 



Skin. Note the color of the skin on the dorsal surface of the 

 frog's body. Compare with the color on the ventral surface. In a 

 later chapter it will be pointed out that many zoologists think that 

 these colors help to conceal the frog in the grass along ponds, among 

 water weeds, and in other places where frogs live. When you make 

 a trip into the country look for evidence that the color helps to con- 

 ceal the frog. The skin is covered with a slime or mucus. How 

 would this help the animal if an enemy tried to catch it ? 



Are there hairs on the frog's skin? Look with a hand-lens. 

 Compare with your own skin. 



Sense-organs. (a) Examine eyes. Are there eyelids ? Can 

 the frog close its eyes ? Look into a small mirror and compare your 

 own eyes with those of a frog. (6) Ears the large, dark round 

 spots just behind the eyes are the membranes of the ears, stretched 

 like a drum-head over the cavities (internal ears), which will be 

 examined later. The frog has no projecting external ears such as 

 the human being has. Also in the human ear the ear-drum (tym- 

 panum) cannot be seen from the outside, because it lies deep in a 

 canal or tube which leads to the internal ear. 



Limbs. Compare the anterior pair of limbs with the posterior 

 ones. Are they alike ? In the anterior limbs (fore legs) notice three 

 divisions : upper arm, extending from shoulder to elbow ; fore- 

 arm, extending from elbow to wrist ; and the "hand." How many 

 "fingers" ? Compare the anterior limb with your own; i.e., your 

 arm. What differences in the divisions do you notice ? What 

 similarities? In the posterior leg (hind limb or hind leg) of the frog 



