But in these organs there is no great or striking change; the time for marked and rapid development of the digestive and reproductive systems has gone by.  Material can be more profitably invested in brain or muscle.  Air is carried to all parts of the body by a special system of air-sacks and tubes.  This is a very advantageous structure for small animals with an external skeleton.  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.  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.