THE CIRCULATION OF THE BLOOD. 



105 



the body instead of being carried by the agency of a cir- 

 culating medium viz., the blood. 



The first Approach to a Circulatory System is made 

 by the Star-fish and the Sea-urchin. A vein runs along 

 the whole length of the alimentary tube, to absorb the 

 chyle, and forms a circle around each end of the tube. 

 These circular vessels send off branches to various parts 

 of the body; but as they are not connected by a net-work 

 of capillaries, there can be no circuit (Fig. 39). 



A higher type is exhibited by the Insects. If we ex- 

 amine the back of any thin-skinned Caterpillar, a long 

 pulsating tube is seen running beneath 

 the skin from one end of tke body to 

 the other. This dorsal vessel, or heart, 

 as it is called, is open at both ends, and 

 divided by valves into compartments, 

 permitting the blood to go forward, 

 but not backward. Each compartment 

 communicates by a pair of slits, guard- 

 ed by valves, with the body - cavity, so 

 that fluids may enter, but cannot es- 

 cape. " Circulation " is very simple. 

 We have seen that the chyle exudes 

 through the walls of the alimentary ca- 

 nal directly into the cavity of the abdo- 

 men, where it mingles with the blood 

 already there. This mixed fluid is 

 drawn into the dorsal tube through the 



& 



valvular openings as it expands; and 

 upon its contraction, all the side-valves 

 are closed, and the fluid is forced tow- 

 ards the head. Passing out at the front 

 opening, it is again diffused among and 

 between the tissues of the body. The blood, therefore, 

 does not describe a circle in definite channels so as to re- 

 turn constantly to its point of departure. 



sal Vessel, or Heart, of 

 a Cockchafer bisected : 

 a, 6, mnscnlar walls; 

 d, valves between the 

 compartments; c, valve 

 defending one of the 

 orifices communicating 

 with the general cavity 

 of the abdomen. 



