Crinoid

Crinoids are marine animals that make up the class Crinoidea, one of the classes of the phylum Echinodermata, which also includes the starfish, brittle stars, sea urchins and sea cucumbers.
 Those crinoids which, in their adult form, are attached to the sea
bottom by a stalk are commonly called sea lilies, while the unstalked
forms are called feather stars or comatulids, being members of the
largest crinoid order, Comatulida.
Adult
 crinoids are characterised by having the mouth located on the 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 fivefold symmetry can be
recognised, in most crinoids the five arms are subdivided into ten or
more. These have feathery pinnules and are spread wide to gather planktonic particles from the water. At some stage in their life, most crinoids have a stem used to attach themselves to the substrate, but many live attached only as juveniles and become free-swimming as adults.
There are only about 600 living species of crinoid, but the class was much more abundant and diverse in the past. Some thick limestone beds dating to the mid- to late-Paleozoic era are almost entirely made up of disarticulated crinoid fragments.

The
 name "Crinoidea" comes from the Greek word κρίνος, "a lily", with the
suffix –oid meaning "like". They live in both shallow water and in
depths as great as 9,000 meters (30,000 ft). Those crinoids which in
their adult form are attached to the sea bottom by a stalk are commonly
called sea lilies. The unstalked forms are called feather stars or
comatulids, being members of the largest crinoid order, Comatulida.

The basic body form of a crinoid is a stem (not present in adult feather
 stars) and a crown consisting of a cup-like central body known as the
theca, and a set of five rays or arms, usually branched and feathery.
The mouth and anus are both located on the upper side of the theca, making the dorsal (upper) surface the oral surface, unlike in the other echinoderm groups such as the sea urchins, starfish and brittle stars
 where the mouth is on the underside. The numerous calcareous plates
make up the bulk of the crinoid, with only a small percentage of soft
tissue. These ossicles fossilise well and there are beds of limestone
dating from the Lower Carboniferous around Clitheroe, England, formed almost exclusively from a diverse fauna of crinoid fossils.
The
 stem of sea lilies is composed of a column of highly porous ossicles
which are connected by ligamentary tissue. It attaches to the substrate
with a flattened holdfast or with whorls of jointed, root-like structures known as cirri.
 Further cirri may occur higher up the stem. In crinoids that attach to
hard surfaces, the cirri may be robust and curved, resembling birds'
feet, but when crinoids live on soft sediment, the cirri may be slender
and rod-like. Juvenile feather stars have a stem, but this is later
lost, with many species retaining a few cirri at the base of the crown.
The majority of living crinoids are free-swimming and have only a vestigial
 stalk. In those deep-sea species that still retain a stalk, it may
reach up to 1 m (3 ft) in length (although usually much smaller), and
fossil species are known with 20 m (66 ft) stems.
The theca is pentamerous (has five-part symmetry) and is homologous
 with the body or disc of other echinoderms. The base of the theca is
formed from a cup-shaped set of ossicles (bony plates), the calyx, while the upper surface is formed by the weakly-calcified tegmen, a mebranous disc.

Feeding
Crinoids are passive suspension feeders, filtering plankton and small particles of detritus
 from the sea water flowing past them with their feather-like arms. The
arms are raised to form a fan-shape which is held perpendicular to the
current. Mobile crinoids move to perch on rocks, coral heads or other
eminences to maximise their feeding opportunities. The food particles
are caught by the primary (longest) tube feet, which are fully extended
and held erect from the pinnules, forming a food-trapping mesh, while
the secondary and tertiary tube feet are involved in manipulating
anything encountered.
The tube feet are covered with sticky mucus that traps any particles which come in contact. Once they have caught a particle of food, the tube feet flick it into the ambulacral
 groove, where the cilia propel the mucus and food particles towards the
 mouth. Lappets at the side of the groove help keep the mucus stream in
place. The total length of the food-trapping surface may be very large;
the 56 arms of a Japanese sea lily
 with 24 cm (9 in) arms, have a total length of 80 m (260 ft) including
 the pinnules. Generally speaking, crinoids living in environments with
relatively little plankton have longer and more highly branched arms than those living in food-rich environments.
The mouth descends into a short oesophagus. There is no true stomach, so the oesophagus connects directly to the intestine, which runs in a single loop right around the inside of the calyx. The intestine often includes numerous diverticulae, some of which may be long or branched. The end of the intestine opens into a short muscular rectum. This ascends towards the anus,
 which projects from a small conical protuberance at the edge of the
tegmen. Faecal matter is formed into large, mucous-cemented pellets
which fall onto the tegmen and thence the substrate.

Most modern crinoids, i.e., the feather stars, are free-moving and lack a
 stem as adults. Examples of fossil crinoids that have been interpreted
as free-swimming include Marsupitsa, Saccocoma and Uintacrinus. In
general, crinoids move to new locations by crawling, using the cirri as
legs. Such a movement may be induced in relation to a change in current
direction, the need to climb to an elevated perch to feed, or because of
 an agonistic behaviour by an encountered individual. Crinoids can also
 swim. They do this by co-ordinated, repeated sequential movements of
the arms in three groups. At first the direction of travel is upwards
but soon becomes horizontal, travelling at about 7 cm (2.8 in) per
second with the oral surface in front. Swimming usually takes place as
short bursts of activity lasting up to half a minute, and in the
comatulid Florometra serratissima at least, only takes place after mechanical stimulation or as an escape response evoked by a predator.
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 could move, before this
recording the fastest motion known for a stalked crinoid was 0.6 metres
(2 feet) per hour. The 2005 recording showed one of these moving across
the seabed at the much faster rate of 4 to 5 cm (1.6 to 2.0 in) per
second (144 to 180 metres per hour).

Origins
If one ignores the enigmatic Echmatocrinus of the Burgess Shale, the earliest known unequivocal crinoid groups date back to the Ordovician.
 There are two competing hypotheses pertaining to the origin of the
group: the traditional viewpoint holds that crinoids evolved from within
 the blastozoans (the eocrinoids and their derived descendants, the blastoids and the cystoids), whereas the most popular alternative suggests that the crinoids split early from among the edrioasteroids.
 The debate is difficult to settle, in part because all three candidate
 ancestors share many characteristics, including radial symmetry,
calcareous plates, and stalked or direct attachment to the substrate.
Diversity
Echinoderms with mineralized skeletons entered the fossil record in the early Cambrian
 (540 mya), and during the next 100 million years, the crinoids and
blastoids (also stalked filter-feeders) were dominant. At that time,
the Echinodermata included twenty taxa of class
 rank, only five of which survived the mass extinction events that
followed. The long and varied geological history of the crinoids
demonstrates how well the echinoderms had adapted to filter-feeding.
The crinoids underwent two periods of abrupt adaptive radiation,
 the first during the Ordovician (485 to 444 mya), and the other during
the early Triassic (around 230 mya). This Triassic radiation resulted
in forms possessing flexible arms becoming widespread; motility,
 predominantly a response to predation pressure, also became far more
prevalent than sessility. This radiation occurred somewhat earlier than
 the Mesozoic marine revolution,
 possibly because it was mainly prompted by increases in benthic
predation, specifically of echinoids. There then followed a selective mass extinction at the end of the Permian.

Crinoidea has been accepted as a distinct clade of echinoderms since the definition of the group by Miller in 1821. According to the World Register of Marine Species, Articulata, the only extant subclass of Crinoidea, includes the following families:-
• order Comatulida Clark, 1908
• order Cyrtocrinida
• order Encrinida †
• order Hyocrinida
• order Isocrinida
• order Millericrinida †
Phylogeny
The phylogeny, geologic history, and classification of the Crinoidea
 was discussed by Wright et al. (2017). These authors presented new
phylogeny-based and rank-based classifications based on results of
recent phylogenetic analyses. Their rank-based classification of
crinoid higher taxa (down to Order), not fully resolved and with
numerous groups incertae sedis (of uncertain placement), is illustrated in the cladogram.

Fossilised crinoid columnal segments extracted from limestone quarried on Lindisfarne, or found washed up along the foreshore, were threaded into necklaces or rosaries, and became known as St. Cuthbert's beads in the Middle Ages. Similarly, in the Midwestern United States, fossilized segments of the columns of crinoids are sometimes known as Indian beads. Crinoids are the state fossil of Missouri.