In very large
animals, or where the skeleton is internal, it would hardly be
practicable; the risk of compression of the tubes at some point, and
of thus cutting off the air-supply of some portion of the body,
would be altogether too great.
The circulatory system is very poor. It consists practically only of
a heart, which drives the blood in an irregular circulation between
the other organs of the body much as with a syringe you might keep
up a system of currents in a bowl of water. But the rapidity of the
flow of the blood in our bodies is mainly to furnish a supply of
oxygen to the organs. A tea-spoonful of blood can carry a fair
amount of dissolved solid nutriment like sugar, it can carry at each
round but a very little gas like oxygen. Hence the blood must make
its rounds rapidly, carrying but a little oxygen at each circuit.
But in the insect the blood conveys only the dissolved solid
nutriment, the food; hence a comparatively irregular circulation
answers all purposes.
The skeleton is a thickening of the horny cuticle of the annelid on
the surface of each segment. The horny cylinder surrounding each
segment is composed of several pieces, and on the abdomen these are
united by flexible, infolded membranes. This allows the increase in
the size of the segment corresponding to the varying size of the
digestive and reproductive systems.
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