THE HEART AND THE ARTERIES. 71 
their larger and smaller branches, which proceed from it 
and ramify through the body, to terminate eventually in 
the blood sinuses, which represent the veins of the 
higher animals. 
When the carapace is removed from the middle of the 
region which lies behind the cervical groove, that is, 
when the dorsal or tergal wall of the thorax is taken 
away, a spacious chamber is laid open which is full of 
blood. This is the cavity already mentioned as the peri- 
cardium (fig. 15, p), though, as it differs in some respects 
from that which is so named in the higher animals, it will 
be better to term it the pericardial sinus. 
The heart (fig. 15, h), lies in the midst of this sinus. It: 
is a thick muscular body (fig. 16), with an irregularly hexa- 
gonal contour when viewed from above, one angle of the 
hexagon being anterior and another posterior. The lateral 
angles of the hexagon are connected by bands of fibrous tis- 
_ sue (ac) with the walls of the pericardial sinus. Otherwise, 
the heart is free, except in so far as it is kept in place by the 
arteries which leave it and traverse the walls of the peri- 
cardium. One of these arteries (figs. 5, 12, and 16, saa), 
starting from the hinder part of the heart, of which it 
is a sort of continuation, runs along the middle line of 
the abdomen above the intestine, to which it gives off 
many branches. A second large artery starts from a 
dilatation, which is common to it with the foregoing, but 
passing directly downwards (figs. 12 and 15, sa, and fig. 16, 
st. a), sither on the right or on the left side of the intestine, 
