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 essential plan, middle thoracic regions of three segments more or
less united, bearing three pairs of legs and usually two pairs of wings,
while the hinder part is a freely jointed abdomen without real limbs. In
these respects the countless varieties of insects agree so that they also
like crustacea of various kinds seem to have been derived from wormlike
animals with more simply segmented bodies. Indeed spiders and scorpions
and their relatives of the group arachnida prove for similar reasons to be