Crinoid

Crinoids are marine invertebrates that make up the class Crinoidea. Crinoids that remain attached to the sea floor by a stalk in their adult form are commonly called sea lilies, while the unstalked forms, called feather stars or comatulids, are members of the largest crinoid order, Comatulida. Crinoids are echinoderms in the phylum Echinodermata, which also includes the starfish, brittle stars, sea urchins, and sea cucumbers. They live in both shallow water and at depths over 9,000 m (30,000 ft).

Adult crinoids are characterised by having their mouths located on their upper surface. This is surrounded by feeding arms and is linked to a U-shaped gut, with the anus being located on the oral disc near the mouth. Although the basic echinoderm pattern of five-fold symmetry can be recognised in most crinoids, the five arms are subdivided into 10 or more. These have feathery pinnules and are spread wide to gather planktonic particles from the water. At some stage in their lives, most crinoids have a short stem used to attach themselves to the substrate, but many live attached only as juveniles and become free-swimming as adults.

Only about 700 living species of crinoids are known, but the class was much more abundant and diverse in the past. Some thick limestone beds dating to the mid-Paleozoic era to Jurassic period are almost entirely made up of disarticulated crinoid fragments.

Etymology

The name "Crinoidea" comes from the Ancient Greek word κρίνον (krínon), "a lily", with the suffix –oid meaning "like".