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. In this part of the body the
skeletal ring of each segment is joined to that of the segments
before and behind it in the same manner. But in other parts of the
body we shall find the skeletal pieces of each segment and the rings
of successive segments fused in one plate of mail. The legs are the
parapodia of annelids carried to a vastly higher development. They
are slender and jointed, and yet often very powerful. A large
portion of the muscular system of the body is attached to these
appendages.

But the insect has also jaws. The annelid had teeth or claws
attached to the proboscis. But true jaws are something quite
different. They always develop by modifying some other organ. In the
insect they are modified legs. This is shown first by their
embryonic development. But the king- or horseshoe-crab has still no
true jaws, but uses the upper joints of its legs for chewing. There
are primitively three pairs of jaws of various forms for the
different kinds of food of different species or higher groups. But
some of them may disappear and the others be greatly modified into
awls for piercing, or a tube for sucking honey. Into the wonderful
transformations of these modified legs we cannot enter.

The muscles are no longer arranged to form a sack as in annelids.
Transverse muscles, running parallel to the unyielding plates of
chitin or horn could accomplish nothing. They have largely
disappeared. The work of locomotion has b