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 and thorax to locomotion and brain. Head and thorax
have increased steadily in importance, while the abdomen has
decreased as steadily in number of segments. And the brain is
increasing thus rapidly because there are now muscles and
sense-organs of sufficient power to make such a brain of value. And
this brain perceives not only objects and qualities, but invisible
relations between these, and this is an advance amounting to a
revolution. It remembers, and uses its recollections. It is capable
of learning a little by experience and observation. The A, B, C of
thinking was probably learned long before the insect's time, and the
bee shows a fair amount of intelligence.