108 
COMPARATIVE ZOOLOGY. 
no great advance in the apparatus of the lowest Verte¬ 
brates over that of the highest Mollusks. The heart of 
Fishes, as in the Oyster, has 
two cavities, but its position 
is reversed. Instead of driv¬ 
ing arterial blood over the 
body, it receives the return¬ 
ing, or venous, blood, and 
sends it to the gills. Re¬ 
collected from the gills, the 
blood is passed into a large 
artery, or aorta , along the 
back, which distributes it by 
a complex system of capil¬ 
laries among the tissues. 
amide; a, aorta, or main artery. These Capillaries Unite with 
the ends of the veins which pass the blood into the auri¬ 
cle 60 (Fig. 48). 
In Amphibians and in Reptiles generally (as Frogs, 
Snakes, Lizards, and Turtles), the heart has three cavities 
—two auricles and one ventricle. The venous blood from 
the body is received into the right auricle, and the puri¬ 
fied blood from the lungs into the left. Both throw their 
contents into the ventricle, which pumps the mixed blood 
in two directions—partly to the lungs, and partly around 
the system. Circulation is, therefore, incomplete, since 
the whole current does not pass through the lungs, and 
three kinds of blood are found in the body—arterial, ve¬ 
nous, and mixed. In many animals, however, arrange¬ 
ments exist which nearly separate the venous from the 
arterial blood. 
The ventricle of Reptiles is partially divided by a par¬ 
tition. In the Crocodile, the division is complete, so that 
there are really four cavities—two auricles, and two ven¬ 
tricles. But both ventricles send off aortas which cross 
