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Fig. 91.—Showing heart and main blood vessels.
The Circulation.—The blood is a liquid containing white corpuscles.
It lacks red corpuscles and is colorless. The heart is in
the upper part of the thorax. It is surrounded
by a large, thin bag, and thus it is
in a chamber (called the pericardial sinus).
The blood from the pulmonary veins enters
this sinus before it enters the heart. The
origin of this pericardial sinus by the fusing of
veins is shown in Fig. 130. Does one artery,
or do several arteries, leave the heart? There
is a larger dorsal artery lying on the intestine
and passing back to the telson; there are
three arteries passing forward close to the
dorsal surface (Figs. 89, 91). One large artery
(the sternal) passes directly downward (Figs.
88, 91), and sends a branch forward and
another backward near the ventral surface.
The openings into the heart from the sinus
have valvular lips which prevent a backward
flow of blood into the sinus. Hence, when
the heart contracts, the blood is sent out into the several
arteries. The arteries take a supply of fresh blood
to the eyes, stomach, muscles, liver, and the various
organs. After it has given oxygen to the several organs
and taken up carbon dioxid, it returns by veins to pass
through the gills on each side, where it gives out the useless
gas and takes up oxygen from the water. It is then
led upward by veins into the pericardial sinus again.
A double nerve chain of ganglia supplies nerve force to the various nerves (Fig. 92). This main nerve chain lies along the ventral surface below the food tube (Fig. 90), except one pair of ganglia which lie above the esophagus or gullet (Fig. 88), and are called the supra-esophageal ganglia, or brain.
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Fig. 92.
Crustacea.—Because of the limy crust which covers the crawfish and its kindred, they are placed in the class called Crustacea.