Crinoids are marine animals that make up the class Crinoidea of the echinoderms (phylum Echinodermata). Crinoidea comes from the Greek word krinon, "a lily", and eidos, "form". They 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.
There are only about 600 existent crinoid species, but they were much more abundant and diverse in the past. Some thick limestone beds dating to the mid- to late-Paleozoic are almost entirely made up of disarticulated crinoid fragments
Most modern crinoids are free-swimming and lack a stem. Examples of fossil crinoids that have been interpreted as free-swimming include Marsupitsa, Saccocoma and Uintacrinus. In 2005, a stalked crinoid was recorded pulling itself along the sea floor off the Grand Bahama Island. While it has been known that stalked crinoids move, before this recording the fastest motion of a crinoid was 0.6 metres/hour (2 ft/h). The 2005 recording showed a crinoid moving much faster.