its lungs. Why? The heart of a fish is near its gills for the same reason. The heart has one auricle and one ventricle. (Colored figure 1.)
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Fig. 221.—Plan of Circulation.
Ab, arteries to gills; Ba, aortic bulb; V, ventricle.
Blood returning to the heart comes through several veins into a
sinus, or antechamber, whence it passes down through a valve
into the auricle; from the auricle it goes forward into the ventricle.
The ventricle sends it into an artery, not directly, but through a
bulb (as, Fig. 220), which serves to maintain
a steady flow, without pulse beats, into the
large artery (aorta) leading to the gills. The
arteries leading from the gills join to form a
dorsal aorta (Ao, Fig. 221), which passes
backward, inclosed by the lower processes of
the spinal column. After going through the
capillaries of the various organs, the blood
returns to the heart through veins.
The color of the blood is given by red corpuscles. These are nucleated, oval, and larger than the blood corpuscles of other vertebrates. The blood of the fish is slightly above the temperature of the water it inhabits.
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Fig. 222.—Brain of Perch, from above.
n, end of nerve of smell; au, eye; v, z, m, fore, mid, and hind brain; h, spinal bulb; r, spinal cord.
Notice the general shape of the brain
(Fig. 222). Are its subdivisions distinct or
indistinct? Are the lobes in pairs? The
middle portion of the brain is the widest,
and consists of the two optic lobes. From these lobes the optic
nerves pass beneath the brain to the eyes (Sn, Fig. 223). In
