102 COMPARATIVE ZOOLOGY. 
CHAPTER XIII. 
THE CIRCULATION OF THE BLOOD. 
The Blood is kept in continual motion in order to 
nourish and purify the body and itself. Jor as life means 
work, and work brings waste, 
there is constant need of 
fresh material to make good the loss in every part of the 
system, and of the removal of matter which is no longer 
fit for use. 
In the very lowest animals, where every part of the 
Aw Me 
Wy y) 
Fig. 66.—Venous Valves. They usually oc- 
cur in pairs, as represented. 
structure is equally capa- 
ble of absorbing the di- 
gested food and is in con- 
tact with it, there is no 
occasion for any circula- 
tion, althongh in them 
even it is not allowed to 
stagnate. But in propor- 
tion as the power of ab- 
sorption is confined to cer- 
tain parts, the more need 
and the greater complexi- 
ty of an apparatus for con- 
veying the nutritious fluid 
to the various tissues. 
In nearly all animals, 
the nutritive fluid is con- 
veyed to the various parts 
of the body by a system 
of tubes, called dlood-ves- 
sels. The higher forms 
