TH?; CIRCULATION. 147 



exterior without having special avenues set apart for carry- 

 ing it. Then on account of the sluggishness of many of 

 the invertebrates, and the further fact that their bodies 

 need not be kept at the relatively high temperature of warm- 

 blooded animals, their need of a rapid circulation does not 

 exist. In such cases, as for instance the clam, the blood 

 circulates in a much slower and less perfect system than in 

 higher forms. 



In the insects an entirely different arrangement is found. 

 The activity which many of this class exhibit would make 

 it imperative to have at the disposal of their tissues abund- 

 ant quantities of oxygen, and if such were to be carried by 

 the blood from some central lung, it would have to run at a 

 speed possibly none slower relatively than that found in us. 

 But oxygen in these little active creatures is not carried by 

 the blood directly, but is carried through special vessels which 

 branch and re-branch, carrying the air from the exterior to 

 every part of the body. In other words, the insects have a 

 system of air arteries and air capillaries permeating the en- 

 tire body to the minutest parts, and by this system sufficient 

 quantities of oxygen are carried. The blood in this case, 

 being relieved of its main function, circulates in a very im- 

 perfect way, and the blood system is not at all well de- 

 veloped. 



Simplified conditions are also met in the lower forms of 

 vertebrates. Thus in fishes the heart consists of but one 

 auricle and one vertricle. In the frogs and allied forms 

 there are two auricles, but one ventricle only, while, finally, 

 m the higher forms we have four distinct cavities of the 

 heart. By this means the blood reaches the heart twice in 

 one complete circulation instead of only once, as in the 

 fishes, and is thus with almost double energy forced on in 

 its course. 



THE GENERAL ARRANGEMENT OF THE CIRCULATORY SYSTEM. 



To have any liquid circulate it is necessary that at one 

 point at least in its course it may be subjected to a proper 



