THE FROG 73 



with the surroundings of this animal and makes it less 

 conspicuous to its enemies. 



How many eyes has the frog, and where are they situated ? 

 Note how they protrude. Touch one of them with a pen- 

 cil. Is the eye withdrawn ? Note that it drops downward 

 and inward. Determine the number of lids. Which is 

 the thinner and capable of more movement? The under 

 one is sometimes called the nictitating membrane. 



Just behind each eye find the tympanic membranes of 

 the ears. Note the white spot in the center of each. This 

 is the place of attachment of the columella (see text, 

 page 230). 



Note the wide, capacious mouth and the fleshy lips bor- 

 dering the jaws. 



Circulation of the blood in the web of the hind foot. Procure 

 a thin board or shingle five or six inches long and near the 

 middle of one end cut a V-shaped opening about the size 

 of the expanded web. Wrap the frog in a wet towel or 

 cloth with a hind foot protruding, and, with tape, tie the 

 animal firmly but not too tightly to the board. Do not 

 tear the web, but stretch it carefully over the opening by 

 attaching threads to the outside toes and tying them to 

 tacks stuck in the board. Then place the whole firmly 

 on the stage of a microscope and examine the web with 

 the low-power objective. 



Note the network of blood vessels in the web. The 

 larger vessels running mainly toward the free ends of the 

 web, and constantly diminishing in size by subdivision, are 

 the arteries. They gradually break up into capillaries. 

 The capillaries unite and form large vessels, the veins. 



Now place a cover glass on the web and run water be- 

 neath it, Examine the web with the high-power objective. 



