The ganglia of the three post-oral segments, which bear the jaws,
have fused in one compound ganglion innervating the mouth and jaws.
Those of the three præ-oral segments have fused to form a brain.
Eyes are well developed, giving images sometimes accurate in detail,
sometimes very rude. Ears are not uncommon. The sense of smell is
often keen.

Perhaps the greatest advance of the insect is its adaptation to land
life. This gives it a larger supply of oxygen than any aquatic
animal could ever obtain. This itself stimulates every function, and
all the work of the body goes on more energetically. Then the heat
produced is conducted off far less rapidly than in aquatic forms.
Water is a good conductor of heat, and nearly all aquatic animals
are cold-blooded. The few which are warm-blooded are protected by a
thick layer of non-conducting fat. In all land animals, even when
cold-blooded, the work of the different systems is aided by the
longer retention of the heat in the body.

Let us recapitulate. The schematic worm had a body composed of two
concentric tubes. The outer was composed of the muscles of the body
covered by the protective integument. The inner tube was the
alimentary canal with its special muscles. Between these two was the
perivisceral cavity, filled with nutritive fluid, lymph, and
furnishing a safe lodging-place for the more delicate viscera. It
represented fairly the trunk of higher animals.

The annelid added segmentation, and thus greater freedom of motion
by the parapodia. But the segments were still practically alike. In
the insect division of labor took place, that is, each group of
segments was allotted its own special work; and these groups of