132 VITAL FUNCTIONS. 



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 its 



Fig. 83. 



movements being indicated by the arrows. The black 

 line along the back is the dorsal vessel ; a, representing 

 the currents in the antennae, w, in the wings, and t, in 

 the tail. In all these parts the vessels form loops deri- 

 ved from the main vessels of the trunk. The currents 

 of blood are unequal in their motions, being accelera- 

 ted by the impulsions they receive from the contractions 

 of the dorsal vessel, which as we have already noticed 

 is the substitute for a heart in these animals. 



CIRCULATION IN THE AMPHIBIA AND FISHES. 



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 artery and vein divided into 

 two trunks each. 



Circulation in the Frog. These parts are represent- 

 ed 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? 



