INSECT CIECTJLATIOK 



213 



the food passes from the digestive tract directly through 

 the walls of the stomach and intestine into the body 

 cavity. Here it would remain were it not for a tube with 

 several swellings along its surface extended along the 

 dorsal surface of the body cavity. This 

 tube, with its swellings, is a sort of 

 muscular pump or heart, closed at the 

 rear end and open in front. The swel- 

 lings expand and contract rhythmi- 

 cally. As they expand the liquid of 

 the body cavity is sucked in through 

 openings along the sides. These open- 

 ings are protected by valves which open 

 inward. As the swellings contract these 

 valves close, and the inclosed fluid is 

 thus forced forward and out the front 

 end of the tube toward the head. In 

 this manner the body fluid is kept in 

 motion, and by means of the cavities 

 between the tissues it flows around 

 and bathes all the tissues, and finally 

 returns to its starting point, thus sup- 

 plying the body cells with food and 

 receiving their wastes. This waste is 

 absorbed by kidney tubes which extend 

 into the body cavity from the intes- 

 tine, and the absorbed fluid is excreted 

 through the intestine with the fffices. 

 The liquid circulated contains no red corpuscles, as the 

 air supply is furnished by an entirely independent set 

 of tubes. There are no arteries or veins, the blood flow- 

 ing through the spaces of the body from the mouth of 



Pig. 77 — Portion of 

 an insect's blood 

 tube (dissected); a, 

 6, muscular walls ; 

 d, valves between 

 the compartments; 

 c, valve covering the 

 general opening into 

 the body cavity; 

 through this valve 

 the liquid enters. 



