COMPARATIVE ZOOLOGY 



There is no true system of blood vessels below the 

 echinoderms. The simplest provision for the distribu- 

 tion of the products of digestion is shown by the jelly- 

 fish, whose stomach sends off radiating tubes (Fig. 21), 

 through which the digested food passes directly to the 

 various parts of the body instead of being carried by 

 the agency of a circulating medium viz., the blood. 



The First Approach to a Circulatory System is made by 

 the starfish 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 network 

 of capillaries, there can be no circuit 

 (Fig. 237). 



A higher type is exhibited by the 

 insects. If we examine the back of 

 any thin-skinned caterpillar, a long 

 pulsating tube is seen running beneath 

 the skin from one end of the 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, guarded 

 by valves, with the body cavity, so that 

 fluids may enter, but cannot escape. 

 " Circulation " is very simple. We have 

 seen that the chyle exudes through the 

 walls of the alimentary canal directly 

 into the cavity of the abdomen, where it mingles with 

 the blood already there. This mixed fluid is drawn 



FIG. 266. Part of the 

 Dorsal Vessel, or 

 Heart, of a Cock- 

 chafer bisected : a, b, 

 muscular walls; rf, 

 valves between the 

 compartments; c, 

 valve defending one 

 of the orifices com- 

 municating with the 

 general cavity of the 

 abdomen. 



