Editing Crinoid (section)

==Morphology==
[[File:Crinoid anatomy.png|thumb|upright=1.15|Anatomy of a stalked crinoid]]
 
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 (anatomy)|dorsal]] (upper) surface the oral surface, unlike in the other echinoderm groups such as the [[sea urchin]]s, [[starfish]] and [[brittle star]]s where the mouth is on the underside.<ref name=OHara>{{cite book|author1=O'Hara, Timothy|author2=Byrne, Maria|title=Australian Echinoderms: Biology, Ecology and Evolution |url=https://books.google.com/books?id=EtkqDwAAQBAJ&pg=PA171 |year=2017 |publisher=Csiro Publishing |isbn=978-1-4863-0763-0 |pages=171–180}}</ref> The numerous calcareous plates make up the bulk of the crinoid, with only a small percentage of soft tissue. These ossicles fossilize well and there are beds of limestone dating from the [[Mississippian (geology)|Lower Carboniferous]] around [[Clitheroe]], England, formed almost exclusively from a diverse fauna of crinoid fossils.<ref name=HessBrett>{{cite book|author1=Hess, Hans |author2=Brett, Carlton E. |author3=Ausich, William I.|author4=Simms, Michael J.|title=Fossil Crinoids|url=https://books.google.com/books?id=TTKhrnw23MkC |year=2002 |publisher=Cambridge University Press|isbn=978-0-521-52440-7 |pages=3–5, 45–46}}</ref>

[[File:Haeckel Crinoidea.jpg|left|thumb|upright|Stalked crinoid drawn by [[Ernst Haeckel]]]]

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 (biology)|holdfast]] or with whorls of jointed, root-like structures known as [[Cirrus (biology)|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 [[Vestigiality|vestigial]] stalk. In those deep-sea species that still retain a stalk, it may reach up to {{convert|1|m|ft|0|abbr=on}} in length (although usually much smaller), and fossil species are known with {{convert|20|m|ft|0|abbr=on}} stems.<ref name=Ruppert>{{Cite book |last1=Ruppert |first1=Edward E. |last2=Fox |first2=Richard S. |last3=Barnes |first3=Robert D. |year=2004 |title=Invertebrate Zoology: A Functional Evolutionary Approach |url=https://archive.org/details/isbn_9780030259821 |url-access=registration |edition=7th |location=Belmont, CA |publisher=Thomson-Brooks/Cole |pages=[https://archive.org/details/isbn_9780030259821/page/916/mode/2up 917–918] |isbn=9780030259821 |oclc=53021401}}</ref>

The theca is [[Symmetry in biology#Pentamerism|pentamerous]] (has five-part symmetry) and is [[Homology (biology)|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 (anatomy)|calyx]], while the upper surface is formed by the weakly-calcified [[tegmen]], a membranous disc. The tegmen is divided into five "ambulacral areas", including a deep groove from which the [[tube feet]] project, and five "interambulacral areas" between them. The mouth is near the centre or on the margin of the tegmen, and [[ambulacral]] grooves lead from the base of the arms to the mouth. The [[anus]] is also located on the tegmen, often on a small elevated cone, in an [[Ambulacral|interambulacral]] area. The theca is relatively small and contains the crinoid's digestive organs.<ref name=Ruppert/>

The arms are supported by a series of articulating ossicles similar to those in the stalk. Primitively, crinoids had only five arms, but in most modern forms these are divided into two at ossicle II, giving ten arms in total. In most living species, especially the free-swimming feather stars, the arms branch several more times, producing up to two hundred branches in total. Being jointed, the arms can curl up. They are lined, on either side alternately, by smaller jointed appendages known as "pinnules" which give them their feather-like appearance. Both arms and pinnules have [[tube feet]] along the margins of the ambulacral grooves. The tube feet come in groups of three of different size; they have no suction pads and are used to hold and manipulate food particles. The grooves are equipped with [[Organelle|cilia]] which facilitate feeding by moving the organic particles along the arm and into the mouth.<ref name=Ruppert/>

<gallery style="text-align:center;" mode="packed">
Image:Crinoide vraie.jpg|Stem, theca and arms of a "true" (stalked) crinoid (family [[Isselicrinidae]])
Image:Comasteridae - Oxycomanthus bennetti-001.jpg|''[[Oxycomanthus bennetti]]'' (comatulid)
Image:Lamprometra palmata hgsus03.JPG|Tegmen of a ''[[Lamprometra palmata]]''. The mouth is located at the center of the 5 feeding grooves, and the anus at the top of the column.
Image:Elegant feather star9.jpg|Close-up on the cirri that allow comatulids to walk and attach themselves
Image:Myzostoma fuscomaculatum at Percys Hole detail.jpg|Close-up on the pinnules of a ''[[Tropiometra carinata]]'' (with parasites ''[[Myzostoma fuscomaculatum]]'')
</gallery>

===Feeding===
[[File:Podia of a red feather star.jpg|thumb|upright|Two arms with pinnules and tube feet outstretched]]
Crinoids are passive [[suspension feeder]]s, 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.<ref name=Ruppert/>

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 [[Metacrinus rotundus|Japanese sea lily]] with  {{convert|24|cm|in|0|abbr=on}} arms, have a total length of {{convert|80|m|ft|-1|abbr=on}} 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.<ref name=Ruppert/>

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 [[Diverticulum|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.<ref name=Ruppert/>

=== Predation ===
Specimens of the sea urchin ''[[Calocidaris micans]]'' found in the vicinity of the crinoid ''[[Endoxocrinus parrae]]'', have been shown to contain large quantities of stem portions in their guts. These consist of articulated ossicles with soft tissue, whereas the local sediment contained only disarticulated ossicles without soft tissue. This makes it highly likely that these sea urchins are [[Predation|predators]] of the crinoids, and that the crinoids flee, offering part of their stem in the process.<ref>{{cite journal |doi=10.1666/07031.1 |title=Urchins in the meadow: Paleobiological and evolutionary implications of cidaroid predation on crinoids |journal=Paleobiology |volume=34 |issue=1 |pages=22–34 |year=2008 |last1=Baumiller |first1=Tomasz K. |last2=Mooi |first2=Rich |last3=Messing |first3=Charles G. |s2cid=85647638 |jstor=20445573|bibcode=2008Pbio...34...22B }}</ref>

Various crinoid fossils hint at possible prehistoric predators. [[Coprolite]]s of both fish and [[cephalopod]]s have been found containing ossicles of various crinoids, such as the pelagic crinoid ''[[Saccocoma]]'', from the [[Jurassic]] [[Lagerstätte|lagerstatten]] [[Solnhofen]],<ref name="Hess">{{cite book |first=Hans |last=Hess |chapter=Upper Jurassic Solnhofen Plattenkalk of Bavaria, German |chapter-url=https://books.google.com/books?id=TTKhrnw23MkC&pg=PA216 |pages=216–24 |editor1-first=Carlton E. |editor1-last=Brett |editor2-first=William I. |editor2-last=Ausich |editor3-first=Michael J. |editor3-last=Simms |year=2003 |title=Fossil Crinoids |publisher=Cambridge University Press |isbn=978-0-521-52440-7}}</ref> while damaged crinoid stems with bite marks matching the toothplates of [[Coccosteidae|coccosteid]] [[placoderms]] have been found in Late [[Devonian]] [[Poland]].<ref>{{cite journal |doi=10.1127/0077-7749/2010/0111 |title=Inferred placoderm bite marks on Devonian crinoids from Poland |journal=Neues Jahrbuch für Geologie und Paläontologie - Abhandlungen |volume=259 |pages=105–12 |year=2011 |last1=Gorzelak |first1=Przemys Law |last2=Rakowicz |first2=Lukasz |last3=Salamon |first3=Mariusz A. |last4=Szrek |first4=Piotr}}</ref> The calyxes of several Devonian to [[Carboniferous]]-aged crinoids have the shells of a snail, ''[[Platyceras]]'', intimately associated with them.<ref>{{cite journal |last1=Brett |first1=Carlton E. |first2=Sally E. |last2=Walker |title=Predators and predation in Paleozoic marine environments |journal=Paleontological Society Papers |volume=8 |year=2002 |pages=93–118 |url=http://www.yale.edu/ypmip/predation/Chapter_05.pdf |access-date=2014-04-06 |archive-url=https://web.archive.org/web/20120813070821/http://www.yale.edu/ypmip/predation/Chapter_05.pdf |archive-date=2012-08-13 |url-status=dead |doi=10.1017/S1089332600001078 }}</ref> Some have the snail situated over the anus, suggesting that ''Platyceras'' was a [[Coprophagia|coprophagous]] commensal, while others have the animal directly situated over a borehole, suggesting a more pernicious relationship.<ref>{{cite journal |hdl=2027.42/75509 |title=Infestation of Middle Devonian (Givetian) camerate crinoids by platyceratid gastropods and its implications for the nature of their biotic interaction |journal=Lethaia |volume=36 |issue=2 |pages=71–82 |year=2003 |last1=Gahn |first1=Forest J. |last2=Baumiller |first2=Tomasz K. |doi=10.1080/00241160310003072|bibcode=2003Letha..36...71G |url=https://deepblue.lib.umich.edu/bitstream/2027.42/75509/1/00241160310003072.pdf |hdl-access=free }}</ref>

===Water vascular system===
Like other echinoderms, crinoids possess a [[water vascular system]] that maintains [[hydraulic]] pressure in the tube feet. This is not connected to external sea water via a [[madreporite]], as in other echinoderms, but only connected through a large number of pores to the [[coelom]] (body cavity). The main fluid reservoir is the muscular-walled ring canal which is connected to the coelom by stone canals lined with calcareous material. The coelom is divided into a number of interconnecting spaces by [[Mesentery (zoology)|mesenteries]]. It surrounds the viscera in the disc and has branches within the stalk and arms, with smaller branches extending into the pinnules. It is the contraction of the ring canal that extends the tube feet. Three narrow branches of the coelom enter each arm, two on the oral side and one aborally, and pinnules. The action of cilia cause there to be a slow flow of fluid (1mm per second) in these canals, outward in the oral branches and inward in the aboral ones, and this is the main means of transport of nutrients and waste products. There is no heart and separate circulatory system but at the base of the disc there is a large blood vessel known as the axial organ, containing some slender blind-ended tubes of unknown function, which extends into the stalk.<ref name=Ruppert/>

These various fluid-filled spaces, in addition to transporting nutrients around the body, also function as both a respiratory and an excretory system. Oxygen is absorbed primarily through the tube feet, which are the most thin-walled parts of the body, with further gas exchange taking place over the large surface area of the arms. There are no specialised organs for excretion while waste is collected by [[phagocyte|phagocytic]] coelomocytes.<ref name=Ruppert/>

===Nervous system===
The crinoid nervous system is divided into three parts, with numerous connections between them. The oral or uppermost portion is the only one [[homology (biology)|homologous]] with the nervous systems of other echinoderms. It consists of a central nerve ring surrounding the mouth, and radial nerves branching into the arms and is sensory in function. Below this lies an intermediate nerve ring, giving off radial nerves supplying the arms and pinnules. These nerves are motor in nature, and control the musculature of the tube feet. The third portion of the nervous system lies aborally, and is responsible for the flexing and movement actions of the arms, pinnules and cirri. This is centred on a mass of neural tissue near the base of the calyx, and provides a single nerve to each arm and a number of nerves to the stalk.<ref name=Ruppert/>

===Reproduction and life cycle===
Crinoids are [[Dioecy|dioecious]], with individuals being either male or female. In most species, the [[gonad]]s are located in the pinnules but in a few, they are located in the arms. Not all the pinnules are reproductive, just those closest to the crown. The [[gamete]]s are produced in genital canals enclosed in genital coeloms. The pinnules eventually rupture to release the [[sperm]] and [[ovum|eggs]] into the surrounding sea water. In certain genera, such as ''[[Antedon]]'', the fertilised eggs are cemented to the arms with secretions from epidermal glands; in others, especially cold water species from Antarctica, the eggs are [[Egg incubation|brooded]] in specialised sacs on the arms or pinnules.<ref name=Ruppert/>

The fertilised eggs hatch to release free-swimming [[Echinoderm#Larval development|vitellaria larvae]]. The bilaterally symmetrical larva is barrel-shaped with rings of [[Cilium|cilia]] running round the body, and a tuft of sensory hairs at the upper pole. While both feeding (planktotrophic) and non-feeding (lecithotrophic) larvae exist among the four other extant echinoderm classes, all present day crinoids appear to be descendants from a surviving clade that went through a [[Population bottleneck|bottleneck]] after the [[Permian–Triassic extinction event|Permian extinction]], at that time losing the feeding larval stage.<ref>{{cite journal |doi=10.1038/sj.hdy.6800866 |pmid=16850040 |title=The active evolutionary lives of echinoderm larvae |journal=Heredity |volume=97 |issue=3 |pages=244–52 |year=2006 |last1=Raff |first1=R A |last2=Byrne |first2=M|doi-access=free }}</ref> The larva's free-swimming period lasts for only a few days before it settles on the bottom and attaches itself to the underlying surface using an adhesive gland on its underside. The larva then undergoes an extended period of [[metamorphosis|metamorphoses]] into a stalked [[Juvenile (organism)|juvenile]], becoming radially symmetric in the process. Even the free-swimming feather stars go through this stage, with the adult eventually breaking away from the stalk.<ref name=Ruppert/>

====Regeneration====
Crinoids are not capable of clonal reproduction as are some [[starfish]] and [[brittle star]]s, but are capable of regenerating lost body parts. Arms torn off by predators or damaged by adverse environmental conditions can regrow, and even the [[Organ (biology)|visceral mass]] can regenerate over the course of a few weeks.<ref name=Ruppert/> The stalk's uppermost segment and the basal plates have the capacity to regenerate the entire crown. Nutrients and other components from the stalk, especially the upper 5 cm, are used in crown regeneration.<ref name=AmemiyaOli>{{cite journal |last1=Amemiya |first1=Shonan |last2=Oji |first2=Tatsuo |title=Regeneration in sea lilies |journal=Nature |date=June 1992 |volume=357 |issue=6379 |page=546-547 |doi=10.1038/357546a0 |bibcode=1992Natur.357..546A |url=https://doi.org/10.1038/357546a0 |language=en |issn=1476-4687|url-access=subscription }}</ref>

Crinoids have been able to regenerate parts since Paleozoic times.<ref name=AmemiyaOli />  These regenerative abilities may be vital in surviving attacks by predatory fish.<ref name=Ruppert/>