104 COMPARATIVE ZOOLOGY. 
of digestion is shown by the Jelly-fish, whose stomach 
sends off radiating tubes. 
The first Approach to a Circulatory System is made 
by the Star-fish and 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 cap- 
illaries, there can’ be no circuit (Fig. 37). 
The next higher type is exhibited by the Articulates. 
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 com- 
partments, permitting the blood to go 
forward, but not backward. Each com- 
partment communicates by a pair of 
slits, guarded by valves, with the body- 
cavity, so that fluids may enter, but can 
not escape. “Circulation” is very sim- 
ple. We have seen that the chyle ex- 
udes through the walls of the aliment- 
ary canal directly into the cavity of the 
abdomen, where it mingles with the 
= blood already there. This mixed fluid 
Fra. 68.—Part of the Dor. is drawn into the dorsal tube through 
sal Vessel, or Heart, of 
a iater Divectods the valvular openings as it expands; 
a, b, muscular walls; 3 : B 
d, valves between the and upon its contraction, all the side- 
compartments; ¢, valve . . 
defending one of the Valves are closed, and the fluid is 
orifices communicating ars . . 
Galnitth® geuéral cavity forced toward the head. Passing out 
Oia SPH omens at the front opening, it is again dif- 
fused among and between the tissues of the body. The 
blood, therefore, does not describe a circle in definite 
