Crinoid

Crinoids are marine echinoderms in the class Crinoidea, commonly known as sea lilies and feather stars.

Crinoids are marine echinoderms in the class Crinoidea, commonly known as sea lilies and feather stars. They are filter-feeding animals with feather-like arms, and they have a fossil record extending back to the Ordovician, or possibly the Cambrian. Living species occur in shallow reefs, deep sea habitats, and other marine environments.

Classification and anatomy

Crinoids are echinoderms, closely related to sea stars, brittle stars, sea urchins, and sea cucumbers. They have a central crown and five or more flexible arms with feathery pinnules. Their skeleton is made of calcium carbonate plates. Many species have a stalk in some life stages or as adults. A distinguishing feature is that the mouth and anus are on the upper surface, making crinoids the only living echinoderms with an upward-facing oral surface.

Feeding and movement

Crinoids are passive suspension feeders that trap plankton and other particles from water currents using their arms and tube feet. Stalked forms (sea lilies) attach to the seafloor, while unstalked forms (feather stars) can move by crawling and, in some species, swimming.

Evolution and fossils

Crinoids first appear in the fossil record by the Ordovician, with some sources suggesting an earlier origin. They were especially diverse in the Palaeozoic and Mesozoic eras. Many extinct forms are important index fossils. Their calcareous skeletons make them important fossils in palaeontology.

Modern diversity and habitat

Living crinoids number roughly 600 to 700 species. They occupy habitats ranging from shallow coral reefs to the deep sea, including very deep ocean settings and ocean trenches. Many crinoids live in deep water, but some are common on coral reefs.