AIR AND LIFE 



145 



Fig. 43. Breathing tubes in 

 insects 



J, the spiracles in the side of the 



body, opening into the tracheae 



t, which branch repeatedly and 



bring air to all the tissues 



live cells beneath the bark get their 

 oxygen supply by way of the lenti- 

 cels (see Fig, 24), which open to the 

 exterior and connect with passages 

 that carry air to the cambium, or 

 growing layer. Since the cells of 

 the plant use comparatively small 

 amounts of oxygen, they can get 

 enough from the air that diffuses 

 slowly through these openings and 

 passages. The carbon dioxid given 

 off by cells diffuses to the exterior 

 along the same paths (Fig. 42). 

 In insects, which use relatively large 

 amounts of oxygen, the cells in the interior of the body get 

 their supplies from very delicate tubes that branch into all parts 

 and connect with the outside through little openings arranged 

 along the sides (see Fig. 43). The movements of the body, 

 by compressing 

 and releasing 

 these tubes, aid 

 in the circula- 

 tion. In some 

 insects, as the 

 common locust, 

 there are rhyth- 

 mic movements 

 that alternately 

 empty and fill 

 the air pipes, 

 thus accelerat- 

 ing the diffusion 

 of oxygen and 

 the removal of 

 carbon dioxid. 



Fig. 44. How the clam breathes 



The water inside the shell is kept in constant circulation by 

 the vibration of cilia which cover the whole surface of the 

 body and the lining of the mantle m. The current of water 

 (indicated by the arrows) flows forward toward the foot /, up 

 past the mouth, and backward over and between the gills g 

 In the gills an exchange of gases takes place between the blood 

 and the flowing water 



