ain material is beginning to be worth
ten devoted to muscle. The muscular system will still continue to
develop, but the brain has begun an almost endless march of
progress. The eye becomes of continually increasing advantage and
importance because it has a capable brain to use it; and brain is a
more and more profitable investment, because it is served by an
ever-improving eye.

[Illustration: 9. MYRMELEO FORMICARIUS. ANT-LION. HERTWIG, FROM
SCHMARDA.
1, adult; 2, larva; 3, cocoon.]

The annelid had hit upon a most advantageous line of development,
which led ultimately to the insect. The study of the insect will
show us clearly the advantages and defects of the annelid plan.
First of all, the insect, like the mollusk, has an external
skeleton. But the skeleton of the mollusk was purely protective, a
hindrance to locomotion. That of the insect is still somewhat
protective, but is mainly, almost purely, locomotive. It is never
allowed to become so heavy as to interfere with locomotion. In the
second place, the insect has three body regions, having each its own
special functions or work. And one of these is a head. The annelid
had two anterior segments differing from those of the rest of the
body; these may, perhaps, be considered as the foreshadowings of a
structure not yet realized; they can only by courtesy be called a
head. Thirdly, the insect has legs. The annelid had fin-like
parapodia, approaching the legs of insects about as closely as the
fins of a fish approach the legs of a mammal. The reproductive and
digestive systems, while somewhat improved, are not very markedly
higher than those of annelids. The excretory system has more work to
perform and reaches a rather higher development.

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