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 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, guard¬ 
ed by valves, with the body-cavity, so 
that fluids may enter, but cannot es¬ 
cape. u 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. 
Fig. 69.—-Part of the Dor¬ 
sal Vessel, or Heart, of 
a Cockchafer bisected: 
a, 6, muscular walls; 
d, valves between the 
compartments; c, valve 
defending one of the 
orifices communicating 
with the general cavity 
of the abdomen. 
