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Crinoids are marine animals also known as "sea lilies" or "feather-stars". Crinoids are Echinoderms. They live both in shallow water and in depths as great as 6000 meters. The are characterized by a mouth and anus on the top surface, surrounded by radiating feeding arms. Although the basic echinoderm pattern of five-fold symmetry can be recognized, most crinoids have many more than five arms. Crinoids usually have a stem used to attach themselves to a substrate, but many live attached only as juveniles and are free swimming as adults. There are only a few hundred known modern forms, but crinoids were much more numerous both in species and numbers of individuals in the past. Some thick limestone beds dating to the mid- to late Paleozoic are made up entirely of disarticulated crinoid fragments.

The earliest known crinoids come from the Ordovician. They are thought to have evolved from primitive echinoderms known as Eocystoids. Confusingly, another early group of echinoderms was named the Eocrinoids, but is currently thought to be an ancestor of blastoids[?] rather than of crinoids.

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