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.
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