302 FIRST YEAR COURSE IN GENERAL SCIENCE 



living prey. Birds and most fishes do not chew food in the 

 mouth, and so the tongue which in many animals is used 

 to move and hold the food in chewing is lacking or is 

 developed only to assist in swallowing.. 



351. Breathing Movements. The mouth of the fish 

 makes certain movements which look like biting, though there 

 is no food present. If we put some grains of colored matter 

 into the water, we can see them being drawn into the mouth 

 and passing out through slit-like openings on the side of the 

 head, back of the mouth. These movements are breath- 

 ing movements which allow 

 the water, containing dis- 

 solved oxygen from the air, 

 to pass over the breathing 

 organs, called gills. 



352. Breathing Organs. 

 The gills of water animals are 

 covered by thin plates, one 

 on each side of the head, 

 called gill covers, which lift 

 at one side to let the water 

 pass out. The gills look like 

 .fine red fringes attached to 

 curved bony frames. In the 

 " threads" of the fine fringes, 

 blood is flowing all the time. 

 It receives oxygen through the 

 membranes by osmosis, and discharges into the water the 

 waste product, carbon dioxide. The work of the gills 

 of water animals is similar to the work done by the lungs 

 of land animals. 



The breathing organs of vertebrate animals that live in 

 air are lungs. They are elastic bags of spongy tissue con- 

 tained in a cavity in the forward or upper part of the body. 

 An air passage leads from the nostrils into the lungs, where 



FIG. 160. Am SAC IN THE LUNGS 



a, an artery; v, a vein; p, air tube. 



The direction of gases passing 

 between the air sac and the blood 

 vessels is shown by arrows. Which 

 represent the oxygen? 



