240 The American Naturalist. [March, 
heart. The impure blood which has passed through the system 
is conveyed to the right auricle, while the left auricle receives the 
oxygenated blood from the lungs. Thus pure and impure blood 
become mixed in the ventricle. There are two aortic arches 
arising from the ventricle, one from the right side and the other 
from the left. The blood coming through the right aortic arch, 
now become the pulmonary artery, flows through the pulmonary 
artery to the lungs, while that entering the left aortic ar is car- 
ried throughout the system (Fig. 5). 
>, ae eA. 
A R D 
FIG. 3.—CIRCULATORY CENTERS OF MOLLUS CA.—A, dorsal and transverse trunks 
worm; B, heart and auricles of Nautilus; C, heart and ‘auricles of Loligo; D, heart 
sea auricles of Ay je E, heart and piopi R of Gastr ropod; v, ventricle; a, auricle; 
s ieg y ran Ga 2 — abdoeni nalis. The arrows show the direction of the current 
The heart of the bird and of mammals consists of two auricles 
and two ventricles. The venous blood is gathered up from all — 
parts of the body and emptied into the right auricle, whence it 
flows through the tricuspid valves into the right ventricle, which 
by its forcible contraction drives the blood through the pulmo- 
nary artery into the lungs. Here the aérated blood is returned to - 
the heart again through the pulmonary veins to the left auricle, 
thence through the mitral valves to the left ventricle, which sends 
it bounding throughout the system. There are in the bird and 
mammal, then, two distinct hearts; the right half, like the heart 
of the fishes, carrying only venous blood, and the left side, like the | 
heart of the Mollusca, carrying only arterial or aérated blood, 
while the heart of the reptile is an intermediate organ between 
the simple apparatus of the fishes and the compound heart of the 
Mammalia (Fig. V1.). 
