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.*
* See the section on Echinoderms, by Professor MacBride, in the
"Cambridge Natural History," I.
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.
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