copic; but in many respects their importance to the
evolutionist surpasses that of the vertebrates. Their structural plans are
far more varied, and they range more widely from higher and relatively
complicated organisms to the unitary one-celled animals. A knowledge of
some of them is essential for our present purpose, which is to learn how
sure is the basis for the principles of relationship and how complete is
the structural evidence of evolution.

Worms are represented in the minds of most people by the common earthworm
or sandworm. The body in either case is made up of a series of segments or
joints which agree closely throughout the animal in external appearance
and in internal constitution. A section of the digestive tract, a pair of
nerve centers, two funnel-like tubes for excretion, and similar blood
vessels occur in each portion.

Precisely similar features are displayed by the crustacea, which seem to
be so different. Every one is familiar with the appearance of lobsters and
crabs. Even in these animals the body is composed of segments, but these
are not like one another, nor are they freely movable throughout the body.
Five are fused in all crustacea to make a head; in lower members of the
order the eight succeeding segments are free, but in the lobster they are
joined together and united with the head. The hinder part of this animal
is a long abdomen whose segments remain more primitive and independent.
But in a crab, the whole plan has been modified by the shortening and
broadening of the head-thorax, and by the reduction of the abdomen, which
is also turned under the anterior part of the body. The internal organic
systems are constructed upon a worm plan with modifications. Nearly every
one of the segments bears one pair of appendages, which can be referred by
their forked nature to the two-parted, oarlike flaps of sandworms, but the
appendages of crustacea have departed from their prototypes in functional
respects and in details of structure. They are variously feelers, jaws,
legs, pincers, and swimming paddles, evolved to serve different purposes,
just as the limbs of the vertebrates we have described have become
variously arms, wings, flippers and paddles in apes, bats, seals, and
whales.

Butterflies, beetles, bees, and grasshoppers seem at first sight to be
entirely different, even though they agree in being more or less
segmented. But all of them have heads with four pairs of appendages of the
same