By Derek Kellogg and Daphne G. Fautin
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).
