Crinoids are marine animals that make up the class Crinoidea of the echinoderms. 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.
The earliest known crinoid groups date to the Ordovician. There are two competing hypotheses pertaining to the origin of the group: the traditional viewpoint holds that crinoids evolved from within the blastozoans (the eocrinoids and their derived descendants the cystoids), whereas the most popular alternative suggests that the crinoids split early from among the edrioasteroids. The debate is difficult to settle, in part because all three candidate ancestors share many characteristics, including radial symmetry, calcareous plates, and stalked or direct attachment to the substrate. Edited from Crinoid