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 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 derivatives of the same original stock, and own cousins of the insects.

In nearly every one of the invertebrate branches we find representatives which interest us chiefly because they appear to have reached their present condition by retrograde evolution.  Barnacles are really crustacea, but they have lost their eyes as well as some other structures that are most useful in animals with a free existence, because they have adopted a fixed mode of life, which has also brought about the