Crinoidea

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Crinoidea is a small class of echinoderms with around 600 species. Many crinoids live in the deep sea, but others are common on coral reefs.

In most extant crinoids, primarily the shallow-water ones, there are two body regions, the calyx and the rays . The calyx is the cup-shaped central portion that lies below the oral surface, which is oriented away from the substrate; most of the organs are found in the calyx. Both the anus and the mouth open on the oral surface, but the anus is easily distinguished by being located atop an ossified cone set peripherally on the oral surface. The rays are long arm-like extensions from the calyx that are used for feeding. Each ray has a lateral row of short branches on either side; these pinnules increase the surface area and trap food. Some deep-sea crinoids have a third body portion, the stalk . It serves to anchor the crinoid to the substrate. The stalk is largely comprised of stacked calcite disks that are common fossils in limestone. Another conspicuous feature of many criniods are long, thin protrusions called cirri . In unstalked crinoids, the cirri are located on the end of the calyx opposite the mouth, and are used by the animal to grasp the substratum. Cirri of stalked crinoids extend from the stalk; they also seem to function in adhesion. The calcitic ossicles of crinoids, as is typical of echinoderms, form an internal skeleton that provides support and protection.

All crinoids are filter feeders. The tube feet to move food particles down the ambulacral groove of a ray toward the mouth. Modified ossicles called lappets that border the ambulacral groove function to close off the groove and prevent damage to the tube feet.

The rays of crinoids are also important for locomotion. By moving their rays up and down through contraction and relaxation of muscles, crinoids are able to swim slowly through the water.

A crinoid's internal anatomy is dominated by organs for digestion and reproduction. The entire digestive system lies within the calyx and is characterized by little more than a mouth and intestine with diverticula. The coelom extends into the rays, where the gonads are located. Nerves occur throughout the animal, but the mass found in the calyx seems to be the center for regeneration of lost body parts. Excretion may be accomplished through small tubes called saccules located near the ambulacral grooves, but the mechanism for this is poorly understood.

Crinoids are gonochoric and brood their young until the embryo develops into a doliolarian larva or a fully formed juvenile crinoid.

All but one of the 9-11 subclasses of crinoids are now extinct and are known only through their sometimes spectacular fossils. Approximately 5,000 species of fossil crinoids are known, with the greatest diversity from the Paleozoic. By the end of the Permian, however, only one lineage seems to have survived. The only surviving subclass of crinoids is the Articulata.

Although crinoids are sometimes amazingly abundant, they appear to have little commercial impact and hardly affect humans in any way.

References:

Hess, H., W.I. Ausich, C.E. Brett, M.J. Simms. (1999) Fossil Crinoids. Cambridge University Press.

Kolzoff, E. N. (1990) Invertebrates. Sauders College Publishing.

Mladenov, P.V., and Chia, F.S. (1983) Development, settling behavior, metamorphosis and pentacrinoid feeding and growth of the feather star Florometra serratissima. Marine Biology 73:319-323.

Tasch, P. (1973) Paleobiology of the Invertebrates: Data Retrieval from the Fossil Record. John Wiley & Sons, Inc.

Contributors

Derek Kellogg (author), Daphne G. Fautin (author).

Glossary

ectothermic

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