Crinoids, also known as "sea lilies" or "feather-stars", are marine animals that make up the class Crinoidea of the echinoderms
(phylum Echinodermata). They live both in shallow water and in depths
as great as 6000 meters. Crinoids are characterized 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 five-fold symmetry can be recognized, 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 a few hundred known modern forms, but crinoids
were much more numerous both in species and numbers in the past. Some
thick limestone beds dating to the mid- to late-Paleozoic are entirely made up of disarticulated crinoid fragments.
The earliest known crinoids come from the Ordovician.
They are thought to have evolved from primitive echinoderms known as
Eocystoids. Confusingly, another early group of echinoderms were also
the Eocrinoids, but that group is currently thought to be an ancestor of blastoids rather than of crinoids. Some fossil crinoids, such as Pentacrinites,
seem to have lived attached to floating driftwood and complete colonies
are often found. Sometimes this driftwood would become waterlogged and
sink to the bottom, taking the attached crinoids with it. The stem of Pentacrinites can be several metres long. Modern relatives of Pentacrinites live in gentle currents attached to rocks by the end of their stem, which is fairly short.
Most modern crinoids are free-swimming and lack a stem. Examples of free-swimming crinoid fossils include Marsupitsa, Saccocoma and Uintacrinus. Many fossils of free-swimming crinoids (such as Pterocoma) are found in the Jurassic-dated Solnhofen limestone of Solnhofen in Germany, and the Cretaceous-dated Niobrara chalk of Kansas contains large numbers of Uintacrinus.
In 2005, a stalked crinoid was recorded pulling itself along the sea floor off the Grand Bahama Island.
While it has been known that stalked crinoids move, prior to this
recording the fastest motion of a crinoid was 0.6 meters/hour (2 ft/h).
The 2005 recording showed a crinoid moving at 140 meters/hour
Crinoids comprise three basic sections; the Stem, Calyx, and the Arms.
The stem is composed of highly porous ossicles which are filled with
muscular tissue. The Calyx contains the crinoid's digestive and
reproductive organs, and the mouth
is located at the top of the dorsal cup, while the anus is located
peripheral to it. The brachials(arms) display pentameral symmetry and
comprise smaller ossicles than the stem and are equipped with cirri which facilitate feeding by moving the the organic media down the arm and into the mouth.
Crinoids reproduce sexually by the males releasing their sperm and the females releasing their eggs
into the current where they will develop into a bottom-dwelling
non-feeding larval stage and then eventually grow a stalk (in the
stalked crinoids), and within 10 to 16 months will be able to
reproduce. In some cases the female of the species has been known to
temporarly brood the larva.
The crinoids have had an eventful geologic history. Once evolved,
they soon spread to a variety of marine habitats. The group as a whole
suffered a major crisis during the Permian period when most of the crinoid forms of the Palaeozoic era died out, with a few surviving into the Triassic period. During the Mesozoic era there was another great radiation of the crinoids with more modern forms possessing flexible arms becoming widespread.
Fossil crinoid stems from Arkansas.
The long and varied geological history of the crinoids demonstrates
how well the echinoderms have adapted to filter-feeding. The fossils of
other stalked filter-feeding echinoderms, such as blastoids, are also
found in the rocks of the Palaeozoic
era. These extinct groups can exceed the crinoids in both numbers and
variety in certain horizons. They were evidently competing with the
crinoids on an equal basis. However, none of these others survived the
crisis at the end of the Permian period.
An abundance of (stemmed) crinoids occurs in the rocks of the Silurian period in the United Kingdom and the eastern United States, the Devonian period of Kentucky, Michigan, Illinois, New York state and the Hunsrück Slates near Bundenbach in the Eifel region of Germany, the Carboniferous period of the United Kingdom, Belgium and Russia, the Mississippian period of Iowa, Indiana, and Alabama, the Pennsylvanian period of the mid-continental United States, the Permian period of the island of Timor, and the Triassic period of Germany. They are the state fossil of Missouri. Some prehistoric crinoids, like Solanocrinus, lacked stems.
Information from Wikepedia - http://en.wikipedia.org/wiki/Feather_star