Crinoids are marine animals that make up the class Crinoidea of the phylum Echinodermata. Crinoidea live both in shallow water and in depths as great as 6,000 metres. Sea lilies refer to the crinoids which, in their adult form, are attached to the sea bottom by a stalk. Feather stars or comatulids refer to the unstalked forms.

Crinoids are characterised by a mouth on the top surface that is surrounded by feeding arms. They have a U-shaped gut, and their anus is located next to the mouth. Although the basic echinoderm pattern of fivefold symmetry can be recognised, 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 become free-swimming as adults.

The crinoids underwent two periods of abrupt adaptive radiation. The first occurred during the Ordovician, the other after a selective mass extinction at the end of the Permian period. The following Triassic radiation gave rise to forms possessing flexible arms. Motility, predominantly a response to predation pressure, also became far more prevalent. After the end-Permian extinction, crinoids never regained the morphological diversity they enjoyed in the Palaeozoic. Instead, they adopted a different suite of ecological strategies open to them from those that had proven so successful in the Paleozoic.

The long and varied geological history of the crinoids demonstrates how well the echinoderms have adapted to filter-feeding. Some thick limestone beds dating to the mid- to late-Palaeozoic are almost entirely made up of disarticulated crinoid fragments.