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
segments were modified in structure to best suit the performance of
this part of the work of the body. The abdomen was least modified
and its eleven segments were devoted to digestion, reproduction, and
excretion--the old vegetative functions. Three segments were united
in the thorax; all their energy was turned to locomotion, and the
insect became thus an exceedingly active, swift animal. The third
body-region, the head, includes six segments, of which three
surrounded the mouth and furnished the jaws, while two more were
crowded or drawn forward in order that their ganglia might be added
to the old supraoesophageal ganglion and form a brain. It is
interesting to note that a form, peripatus, still exists which
stands almost midway between annelids and insects and has only four
segments in the head. The formation of the head was thus a gradual
process, one segment being added after another.

In the turbellaria the dominant functions were digestion and
reproduction, and their organs composed almost the whole body. Here
only eleven segments at most are devoted to these functions, and
nine in head a