Crinoids (Crinoidea) or commonly known as sea lilies, are a class belonging to the phylum of Echinoderms. Its name derives from the Greek krinon meaning lily and eidos meaning shape, due to the appearance of its branched arms. Crinoids are considered to be the oldest evolutionary group within the Echinoderms.

There are about 625 species of crinoids described, which we can divide into two large groups. On the one hand, the pedunculated crinoids (80 species) that possess in the inferior part of the corporal disc some calcareous elongated appendices, called cirri, that are used to anchor to the substrate. The second group of crinoids (comatulids) are the majority (approximately 540 species) lacking cirri at the base of the body, and have the ability to move slowly through the bottom. Although they have the ability to move, they are usually species that remain attached in one place for long periods of time.

The body of crinoids has a cup-shaped disc (calyx) covered by a tanned skin in which there are calcareous plates. On the upper part of the disc, we find 5 main arms, which branch out repeatedly and which have lateral pinnacles arranged like the teeth of a comb. With the help of the pinnacles of the arms and a secretion of mucous substances, crinoids are able to capture small prey from the waters on which they feed, such as foraminifera, small Crustaceans, Molluscs, zooplankton and phytoplankton.

Like many species of the other Echinoderms groups, crinoid species often have the ability to deliberately amputate one or more of their arms as response to a dangerous situation. These lost arms are completely regenerated, thus restoring their bodily integrity.

They are distributed throughout all seas except the Black and Baltic Seas and in a wide range of depths. Generally, the pedunculated species usually live at depths less than 200 meters, while the non-pedunculated species can live even in the abyssal depths.