The =brittle stars= resemble the starfish in form, but their rays are very slender, more distinct from the disk, and the tube feet are on the edges of the rays, not under them (Fig. 66). 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_). [Illustration: FIG. 67.--CRINOID, arms closed.] [Illustration: FIG. 68.--DISK OF CRINOID from above, showing mouth in center and vent near it, at right (arms removed).] Comparative Review Make a table like this as large as the page of the notebook will allow, and fill in without guessing. ===================+=========+========+=========+=========+========= | AMEBA | SPONGE | HYDRA | CORAL | STARFISH | | | | POLYP | -------------------+---------+--------+---------+---------+--------- | | | | | Is body round, two-| | | | | sided, or irregular| | | | | | | | | | -------------------+---------+--------+---------+---------+--------- | | | | | What organs of | | | | | sense | | | | | | | | | | -------------------+---------+--------+---------+---------+--------- | | | | | Openings into body | | | | | | | | | | | | | | | -------------------+---------+--------+---------+---------+--------- | | | | | Hard or supporting | | | | | parts of body | | | | | | | | | | -------------------+---------+--------+---------+---------+--------- | | | | | How food is taken | | | | | | | | | | | | | | | -------------------+---------+--------+---------+---------+--------- | | | | |