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.
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