The starfish
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-like structure.
Another branch of the family then abandoned the stalk, and,
spreading its arms flat, and gradually developing in them numbers
of little "feet" (water-tubes), became the starfish. In the
living Comatula we find a star passing through the stalked stage
in its early development, when it looks like a tiny sea-lily. The
sea-urchin has evolved from the star by folding the arms into a
ball.
The Bryozoa (sea-mats, etc.) are another and lower branch of the
primitive active organisms which have adopted a sessile life. In
the shell-fish, on the other hand, the principle of
armour-plating has its greatest development. It is assuredly a
long and obscure way that leads from the ancestral type of animal
we have been describing to the headless and shapeless mussel or
oyster. Such a degeneration is, however, precisely what we should
expect to find in the circumstances. Indeed, the larva, of many
of the headless Molluscs have a mouth and eyes, and there is a
very common type of larva--the trochosphere--in the Mollusc world
which approaches the earlier form of some of the higher worms.
The Molluscs, as we shall see, provide some admirable
illustrations of the process of evolution. In some of the later
fossilised specimens (Planorbis, Paludina, etc.) we can trace the
animal as it gradually passes from one species to another. The
freshening of the Caspian Sea, which was an outlying part of the
Mediterranean quite late in the geological record, seems to have
evolved several new genera of Molluscs.
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