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The crinoids are the most ancient of the echinoderms. (Figs. 67, 68.) Their fossils are very abundant in the rocks. They inhabited the geological seas, and it is believed that the other echinoderms descended from them. A few now inhabit the deep seas. Some species are fixed by stems when young, and later break away and become free-swimming, others remain fixed throughout life.

The four classes of the branch echinoderms are Starfish (asteroids), Sea urchins (echinoids), Sea cucumbers (holothurians), and Sea lilies (crinoids).

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Fig. 67.—Crinoid, arms closed.

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Fig. 68.—Disk of Crinoid from above, showing mouth in center and vent near it, at right (arms removed).

Comparative Review

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