AN INTRODUCTION TO ANIMAL BIOLOGY 35 



end of a small tube (blow-pipe) into the glottis and blow in air to 

 inflate the lungs ; or, using a rubber-bulbed pipette, fill them with 

 water. 



(L) Watch a living frog in a glass jar, and observe how the floor 

 of the mouth ("throat") moves up and down at regular intervals. 

 Count the movements in one minute. The frog cannot breathe as we 

 do and practically "swallows" air, forcing it down into the lungs by 

 moving the floor of the mouth up while the mouth and nostrils are 

 closed. Many persons breathe while keeping the mouth open, 

 but a frog cannot breathe when the mouth is held open. Not every 

 movement of the throat forces air down into the lungs; most of 

 the movements simply pump air into and out of the mouth-cavity 

 through the nostrils. But watch carefully, and occasionally the 

 sides (flanks) of the body back of the shoulders will be seen to expand 

 greatly when the frog seems to take a big "swallow" of air; this 

 expansion indicates that air has been forced down into the lungs. 

 Then, after a time, the muscular walls of the body help the elastic 

 lungs expel the air, forcing it up into the mouth-cavity, where it is 

 gradually mixed with fresh air pumped in through the nostrils. 



(D) The kidneys are flat bodies of a dark-red color. They are 

 attached to the dorsal body-wall in the posterior portion of the body- 

 cavity, one on each side of the backbone. The ureters are the ducts 

 or tubes of the kidneys which lead to the terminal part of the large 

 intestine (cloaca) ; they are very small and difficult to demonstrate. 

 The kidneys extract water and certain other substances from the 

 blood which flows through them, and then pass these substances to 

 the exterior through the ureters and cloaca. Before these sub- 

 stances from the kidneys are eliminated, they may be stored for a 

 time in a sac known as the bladder, which lies on the ventral side of 

 the cloaca, and opens into it. This structure is usually found as a 

 collapsed membrane if one looks carefully before cutting out the 

 large intestine. Examine a museum specimen prepared to show the 

 bladder expanded. 



The spleen is a small, oval, red body attached to the mesentery 

 near the large intestine. Its use will be explained after we have 

 studied the blood and lymph. 



Reproductive Organs. (D or L) Examine the rgans in specimens 

 of each sex. In the male frog the spermaries are a pair of yellow 

 ovoid bodies attached ventrally to the kidneys. Their ducts lead 

 into the kidneys, and there connect with little tubes leading to the 

 ureters, and thence through the cloaca to the exterior. 



In the female, the ovaries are attached at about the same place. 



