thers
defensive armour is chiefly developed, and we get the lines of the
heavy sluggish shell-fish, the Molluscs and Brachiopods, and, by a
later compromise between speed and armour, the more active tough-coated
Arthropods. In others the plant-principle reappears; the worm-like
creature retires from the free-moving life, attaches itself to a
fixed base, and becomes the Bryozoan or the Echinoderm. To trace the
development of these types in any detail is impossible. The early
remains are not preserved. But some clues are found in nature or in
embryonic development, and, when the types do begin to be preserved in
the rocks, we find the process of evolution plainly at work in them. We
will therefore say a few words about the general evolution of each type,
and then return to the geological record in the Cambrian rocks.

The starfish, the most familiar representative of the Echinoderms,
seems very far removed from the kind of worm-like ancestor we have been
imagining, but, fortunately, the very interesting story of the starfish
is easily learned from the geological chronicle. Reflect on the
flower-like expansion of its arms, and then imagine it mounted on a
stalk, mouth side upward, with those arms--more tapering than they
now are--waving round the mouth. That, apparently, was the past of the
starfish and its cousins. We shall see that the earliest Echinoderms we
know are cup-shaped structures on stalks, with a stiff, limy frame and
(as in all sessile animals) a number of waving arms round the mouth.
In the next geological age the stalk will become a long and flexible
arrangement of muscles and plates of chalk, the cup will be more
perfectly compacted of chalky plates, and the five arms will taper and
branch until they have an almost feathery appearance; and the animal
will be considered a "sea-lily" by the early geologist.

The evidence suggests that both the free-moving and the stalked
Echinoderms descend from a common stalked Archaean ancestor. Some
primitive animal abandoned the worm-like habit, and attached itself,
like a polyp, to the floor. Like all such sessile animals, it developed
a wreath of arms round the open mouth. The "sea-cucumber" (Holothurian)
seems to be a type that left the stalk, retaining the little wreath of
arms, before the body was heavily protected and deformed. In the others
a strong limy skeleton was developed, and the nerves and other organs
were modified in adaptation to the bud-like or flower-