_Muscular System._--The muscles in the Hexapoda are striated, as in
Arthropods generally, the large fibres being associated in bundles
which are attached from point to point of the cuticle, so as to move
adjacent sclerites with respect to one another (see figs. 8, 10). For
example, the contraction of the tergo-sternal muscles, connecting the
dorsal with the ventral sclerites of the abdomen, lessens the capacity
of the abdominal region, while the contraction of the powerful muscles
arising from the thoracic walls, and inserted into the proximal ends
of the thighs, flexes or extends the legs.
_Circulatory System._--Insects afford an excellent illustration of the
remarkable type of blood-system characterizing the Arthropoda. The
dorsal vessel is an elongate tube, whose abdominal portion is usually
chambered, forming a contractile heart (fig. 10). At the constrictions
between the chambers are paired slits, through which the blood passes
from the surrounding pericardial sinus. The dorsal vessel is prolonged
anteriorly into an aorta, through which the blood is propelled into
the great body-cavity or haemocoel. After bathing the various tissues
and organs, the blood returns dorsalwards into the pericardial sinus
through fine perforations of its floor, and so makes its way into the
heart again. Some water-bugs, e.g. of the families _Belostomatidae_,
_Nepidae_, _Corixidae_ and _Hydrometridae_ have a pulsating sac at
each knee-joint to assist the flow of blood through the legs, while in
dragon-flies and locusts (_Acridiidae_) there is a ventral pulsating
diaphragm, which forms the roof of a sinus enclosing the nerve-cords.
[Illustration: After Miall and Denny, _The Cockroach_, Lovell Reeve &
Co.