Like other echinoderms, crinoids have internal skeletons made of hard “ossicles.” The stalk that anchors a crinoid to a reef or other hard surface is strengthened by stacked, disk-shaped ossicles. The delicate rays of a crinoid are supported by smaller, linked ossicles that provide sites of attachment for the muscles that move them. What remains in the fossil record are these calcified, skeletal parts. Large berms of fossilized skeletons of crinoids are testimony to their abundance during the Paleozoic and Mesozoic eras. Scientists believe that the huge volume of calcified skeletal material left behind by crinoids shaped the environment for other species. Which crinoid fossils are present indicates the age of a fossil bed, since many species lived on Earth for short periods (in geologic time). Despite their prehistoric abundance, only one subclass of crinoids is still living today.

Echinoderms stand out as the only organisms on Earth with five arms or other elements spaced evenly around a central point. This pentaradial symmetry (penta = “five;” radial = “around a central point") is obvious in some echinoderms, such as sea stars or brittle stars that have five arms. It’s more subtle on others, such as the five rows of tube feet on a sea cucumber or five grooves on a sand dollar. This unusual symmetry of adult echinoderms is not found in juveniles. In fact, echinoderm larvae have two-sided (bilateral) symmetry like humans, and must undergo a metamorphosis to become pentaradial adults. Body plans of animals during their development often say something about their ancestry. The bilateral bodies of developing echinoderms are a reminder that, aside from other back-boned animals (vertebrates), echinoderms may be our closest relatives. Unlike most living echinoderms, many fossil echinoderms did not have pentaradial symmetry.