128 VITAL FUNCTIONS. 



381. In the ephemera marginata, a little four-winged 

 fly, the motions of the fluid are quite distinct, and the 

 course it takes is represented by Fig. 83, the direction of 



Fig. 83. 



its movements being indicated by the arrows. The black 

 line along the back is the dorsal vessel ; a representing 

 the currents in the antenna?, w in the wings, and t, in the 

 tail. In all these parts the vessels form loops derived from 

 the main vessels of the trunk. The currents of blood are 

 unequal in their motions, being accelerated by the impul- 

 sions they receive from the contractions of the dorsal ves- 

 sel, which, as we have already noticed, is the substitute 

 for a heart in these animals. 



CIRCULATION IN THE AMPHIBIA AND FISHES. 



382. The most simple apparatus for the circulation of 

 the blood in an air-breathing animal, consists of a single 

 auricle, a single ventricle, with two arteries, and two 

 veins, or rather with a single arteiy and vein divided into 

 two trunks each. 



383. Circulation in the Frog. These parts are repre- 

 sented as they exist in the frog, by Fig. 84, where d is 

 the auricle ; e, the ventricle \ a, the large artery which 

 divides and sends a branch to r ; c, the great vein, called 

 the vena cava, which, like the great artery, divides and also 



Explain the course of the circulation as it takes place in insects. 



