398 GENERAL ZOOLOGY. 



circulatory organs present, one of the functions of which is 

 the carrying of the dissolved food from the digestive tract 

 to the working parts. These circulatory organs are tubes 

 through which the fluid flows, but a flow can only be 

 produced by some mechanism which shall propel the 

 fluid. In most cases this is effected by muscles in the 

 walls of the vessels, which by waves of contraction force 

 the fluid along. In the lower worms and even in 

 Amphioxus (p. 289) all of the vessels are markedly con- 

 tractile and no part has supremacy over another. As 

 we go higher in the scale the tendency is constantly 

 towards a concentration of these pumping muscles in 

 one region, and thus a heart results. 



So far we have traced the fuel to the working parts. 

 In order to do work the fuel must be oxidized, and this 

 means that oxygen must also be brought to these parts. 

 This oxygen is found either in the air or dissolved in the 

 water in which the animal lives. In the Coelenterates, 

 flatworms, and many other forms, the general surface of 

 the body is sufficient for the absorption of the oxygen, 

 but where the animal is larger and needs more oxygen 

 special provisions are needed. 



A very simple condition, physiologically, is found in the 

 insects, where air-tubes (tracheae, p. 239) extend inwards 

 from the outside, their fine branches reaching to every 

 part of the body. Air is drawn into these tubes by an 

 enlargement of the body by suitable muscles, and then, 

 when the oxygen is absorbed, contraction forces out the 

 remainder. This breathing process can be seen by watch- 

 ing the abdomen of a grasshopper or a wasp. So far the 

 circulating fluid is largely nutrient in character, as it 

 carries only the food (and some of the waste) . 



In many Crustacea, in molluscs, worms, and vertebrates, 



