Editing Echinoderm (section)

==Anatomy and physiology==
Echinoderms evolved from animals with [[symmetry (biology)#Bilateral symmetry|bilateral symmetry]]. Although adult echinoderms possess [[symmetry (biology)#Animals|pentaradial]]<!--, or five-sided,--> symmetry, their larvae are [[cilia]]ted, free-swimming organisms with bilateral symmetry<!--, like embryonic [[chordate]]s-->. Later, during metamorphosis, the left side of the body grows at the expense of the right side, which is eventually absorbed. The left side then grows in a [[Pentaradial symmetry#Pentamerism|pentaradially]] symmetric fashion, in which the body is arranged in five parts around a central axis.<ref name=Ruppert2004p873>{{harvnb|Ruppert|Fox|Barnes|2004|p=873}}</ref> Within the [[Asterozoa]], there are a few exceptions from the rule. Most starfish in the genus ''[[Leptasterias]]'' have six arms, although five-armed individuals can occur. The [[Brisingida]] also contain some six-armed species. Amongst the brittle stars, six-armed species such as ''Ophiothela danae'', ''[[Ophiactis savignyi]]'', and ''Ophionotus hexactis'' exist, and ''Ophiacantha vivipara'' often has more than six.<ref name="Byrne OHara 2017">{{harvnb|Byrne|O'Hara|CSIRO|2017|pp=322–324}}</ref>

Echinoderms have secondary radial symmetry in portions of their body at some stage of life, most likely an adaptation to a sessile or slow-moving existence.<ref name="Holló Novák 2012">{{harvnb|Holló|Novák|2012}}</ref> Many crinoids and some seastars are symmetrical in multiples of the basic five; starfish such as ''[[Labidiaster annulatus]]'' possess up to fifty arms, while the [[sea-lily]] ''[[Comaster schlegelii]]'' has two hundred.<ref name="Messing">{{harvnb|Messing|2004}}</ref>

Genetic studies have shown that genes directing anterior-most development are expressed along ambulacra in the center of starfish rays, with the next-most-anterior genes expressed in the surrounding fringe of tube feet.  Genes related to the beginning of the trunk are expressed at the ray margins, but trunk genes are only expressed in interior tissue rather than on the body surface.  This means that a starfish body can more-or-less be considered to consist only of a head.<ref>{{harvnb|Formery|Peluso|Kohnle|Malnick|2023}}</ref><ref>{{harvnb|Lacali|2023}}</ref>

===Skin and skeleton===

Echinoderms have a [[mesoderm]]al skeleton in the dermis, composed of [[calcite]]-based plates known as [[Ossicle (echinoderm)|ossicles]]. If solid, these would form a heavy skeleton, so they have a sponge-like porous structure known as stereom.<ref>{{harvnb|Behrens|Bäuerlein|2007|p=393}}</ref><ref>{{harvnb|Brusca|Moore|Shuster|2016|pp=979–980}}</ref> Ossicles may be fused together, as in the [[Test (biology)|test]] of sea urchins, or may [[Articulation (anatomy)|articulate]] to form flexible joints as in the arms of sea stars, brittle stars and crinoids. The ossicles may bear external projections in the form of spines, granules or warts and they are supported by a tough [[Epidermis (zoology)|epidermis]]. Skeletal elements are sometimes deployed in specialized ways, such as the chewing organ called "[[Aristotle's lantern]]" in sea urchins, the supportive stalks of crinoids, and the structural "lime ring" of sea cucumbers.<ref name=Ruppert2004p873 />

Although individual ossicles are robust and fossilize readily, complete skeletons of starfish, brittle stars and crinoids are rare in the fossil record. On the other hand, sea urchins are often well preserved in chalk beds or limestone. During fossilization, the cavities in the stereom are filled in with calcite that is continuous with the surrounding rock. On fracturing such rock, [[paleontology|paleontologists]] can observe distinctive cleavage patterns and sometimes even the intricate internal and external structure of the test.<ref>{{harvnb|Davies|1925|pp=240–241}}</ref>

The epidermis contains pigment cells that provide the often vivid colours of echinoderms, which include deep red, stripes of black and white, and intense purple.<ref name="Perillo Oulhen 2020">{{harvnb|Perillo|Oulhen|Foster|Spurrell|2020}}</ref> These cells may be light-sensitive, causing many echinoderms to change appearance completely as night falls. The reaction can happen quickly: the sea urchin ''[[Centrostephanus longispinus]]'' changes colour in just fifty minutes when exposed to light.<ref>{{harvnb|Weber|Dambach|1974}}</ref>

One characteristic of most echinoderms is a special kind of tissue known as [[catch connective tissue]]. This [[collagen]]-based material can change its mechanical properties under nervous control rather than by muscular means. This tissue enables a starfish to go from moving flexibly around the seabed to becoming rigid while prying open a [[Bivalvia|bivalve mollusc]] or preventing itself from being extracted from a crevice. Similarly, sea urchins can lock their normally mobile spines upright as a defensive mechanism when attacked.<ref>{{harvnb|Motokawa|1984}}</ref><ref>{{harvnb|Brusca|Moore|Shuster|2016|p=980}}</ref>

===The water vascular system===
{{Main|Water vascular system}}

[[File:FMIB 52615 Diagram of water-vascular system of a starfish ;.jpeg|thumb|Diagram of [[water vascular system]] of a starfish, showing the ring canal, the radial canals, ampullae (small bulbs), and [[tube feet]] ]]

Echinoderms possess a unique water vascular system, a network of fluid-filled canals modified from the [[coelom]] (body cavity) that function in gas exchange, feeding, sensory reception and locomotion. This system varies between different classes of echinoderm but typically opens to the exterior through a sieve-like [[madreporite]] on the aboral (upper) surface of the animal. The madreporite is linked to a slender duct, the stone canal, which extends to a ring canal that encircles the mouth or [[Esophagus#Invertebrates|oesophagus]]. The ring canal branches into a set of radial canals, which in asteroids extend along the arms, and in echinoids adjoin the test in the ambulacral areas. Short lateral canals branch off the radial canals, each one ending in an ampulla. Part of the ampulla can protrude through a pore (or a pair of pores in sea urchins) to the exterior, forming a podium or [[tube feet|tube foot]]. The water vascular system assists with the distribution of nutrients throughout the animal's body; it is most visible in the tube feet which can be extended or contracted by the redistribution of fluid between the foot and the internal ampulla.<ref>{{harvnb|Dorit|Walker|Barnes|1991|pp=780–791}}</ref><ref>{{harvnb|Brusca|Moore|Shuster|2016|pp=980–982}}</ref>

The organisation of the water vascular system is somewhat different in ophiuroids, where the madreporite may be on the oral surface and the podia lack suckers.<ref>{{harvnb|Dorit|Walker|Barnes|1991|pp=784–785}}</ref> In holothuroids, the system is reduced, often with few tube feet other than the specialised feeding tentacles, and the madreporite opens on to the coelom. Some holothuroids like the Apodida lack tube feet and canals along the body; others have longitudinal canals.<ref>{{harvnb|Brusca|Moore|Shuster|2016|p=982}}</ref> The arrangement in crinoids is similar to that in asteroids, but the tube feet lack suckers and are used in a back-and-forth wafting motion to pass food particles captured by the arms towards the central mouth. In the asteroids, the same motion is employed to move the animal across the ground.<ref>{{harvnb|Dorit|Walker|Barnes|1991|pp=790–793}}</ref>

===Other organs===

Echinoderms possess a simple digestive system which varies according to the animal's diet. Starfish are mostly carnivorous and have a mouth, oesophagus, two-part stomach, intestine and rectum, with the anus located in the centre of the aboral body surface. With a few exceptions, the members of the order [[Paxillosida]] do not possess an anus.<ref>{{harvnb|Jangoux|1982|p=244}}</ref><ref>{{harvnb|Rigby|Iken|Shirayama|2007|p=44}}</ref> In many species of starfish, the large cardiac stomach can be everted to digest food outside the body. Some other species are able to ingest whole food items such as [[mollusc]]s.<ref>{{harvnb|Ruppert|Fox|Barnes|2004|p=885}}</ref> Brittle stars, which have varying diets, have a blind gut with no intestine or anus; they expel [[food waste]] through their mouth.<ref>{{harvnb|Ruppert|Fox|Barnes|2004|p=891}}</ref> Sea urchins are herbivores and use their specialised mouthparts to graze, tear and chew their food, mainly [[algae]]. They have an oesophagus, a large stomach and a rectum with the anus at the apex of the test.<ref>{{harvnb|Ruppert|Fox|Barnes|2004|pp=902–904}}</ref> Sea cucumbers are mostly [[detritivore]]s, sorting through the sediment with modified tube feet around their mouth, the buccal tentacles. Sand and mud accompanies their food through their simple gut, which has a long coiled intestine and a large [[cloaca]].<ref>{{harvnb|Ruppert|Fox|Barnes|2004|p=912}}</ref> Crinoids are [[suspension feeder]]s, passively catching [[plankton]] which drift into their outstretched arms. Boluses of mucus-trapped food are passed to the mouth, which is linked to the anus by a loop consisting of a short oesophagus and longer intestine.<ref>{{harvnb|Ruppert|Fox|Barnes|2004|p=920}}</ref>

The [[body cavity|coelomic cavities]] of echinoderms are complex. Aside from the water vascular system, echinoderms have a [[circulatory system|haemal coelom]], a peri[[visceral]] coelom, a [[gonad]]al coelom and often also a perihaemal coelom.<ref>{{harvnb|Moore|Overhill|2006|p=245}}</ref> During development, echinoderm coelom is divided into the metacoel, mesocoel and protocoel (also called somatocoel, hydrocoel and axocoel, respectively).<ref>{{harvnb|Hickman|Roberts|Larson|2003|p=271}}</ref> The water vascular system, haemal system and perihaemal system form the tubular coelomic system.<ref name="wbd.etibioinformatics.nl">{{harvnb|Macrobenthos of the North Sea|loc=Echinodermata}}</ref> Echinoderms are unusual in having both a coelomic circulatory system (the water vascular system) and a haemal circulatory system, as most groups of animals have just one of the two.<ref>{{harvnb|Nielsen|2012|p=78}}</ref>

Haemal and perihaemal systems are derived from the original coelom, forming an [[open circulatory system|open]] and reduced circulatory system. This usually consists of a central ring and five radial vessels. There is no true [[heart]], and the blood often lacks any respiratory pigment. Gaseous exchange occurs via dermal branchiae or papulae in starfish, genital bursae in brittle stars, peristominal gills in sea urchins and cloacal trees in sea cucumbers. Exchange of gases also takes place through the tube feet. Echinoderms lack specialized excretory (waste disposal) organs and so [[nitrogenous waste]], chiefly in the form of [[ammonia]], diffuses out through the respiratory surfaces.<ref>{{harvnb|Dorit|Walker|Barnes|1991|pp=780–791}}</ref>

The coelomic fluid contains the [[coelomocyte]]s, or immune cells. There are several types of immune cells, which vary among classes and species. All classes possess a type of [[Phagocytosis|phagocytic]] amebocyte, which engulf invading particles and infected cells, aggregate or clot, and may be involved in [[cytotoxicity]]. These cells are usually large and granular, and are believed to be a main line of defence against potential pathogens.<ref name=":0">{{harvnb|Ramirez-Gomez|2010}}</ref> Depending on the class, echinoderms may have [[spherule]] cells (for cytotoxicity, inflammation, and anti-bacterial activity), vibratile cells (for coelomic fluid movement and clotting), and crystal cells (which may serve for [[osmoregulation]] in sea cucumbers).<ref name=":0" /><ref>{{harvnb|Smith|2010}}</ref> The coelomocytes secrete [[antimicrobial peptides]] against bacteria, and have a set of [[lectin]]s and [[Complement system|complement proteins]] as part of an [[innate immune system]] that is still being characterised.<ref name="Smith Ghosh Buckley 2010">{{harvnb|Smith|Ghosh|Buckley|Clow|2010}}</ref>

Echinoderms have a simple radial [[nervous system]] that consists of a modified [[nerve net]] of interconnected neurons with no central [[brain]], although some do possess [[ganglion|ganglia]]. Nerves radiate from central rings around the mouth into each arm or along the body wall; the branches of these nerves coordinate the movements of the organism and the synchronisation of the tube feet. Starfish have sensory cells in the epithelium and have simple [[Ocelli|eyespots]] and touch-sensitive tentacle-like tube feet at the tips of their arms. Sea urchins have no particular sense organs but do have [[statocyst]]s that assist in gravitational orientation, and they too have sensory cells in their epidermis, particularly in the tube feet, spines and [[pedicellariae]]. Brittle stars, crinoids and sea cucumbers in general do not have sensory organs, but some burrowing sea cucumbers of the [[Order (biology)|order]] [[Apodida]] have a single statocyst adjoining each radial nerve, and some have an eyespot at the base of each tentacle.<ref>{{harvnb|Ruppert|Fox|Barnes|2004|pp=872–929}}</ref>

The [[gonad]]s at least periodically occupy much of the body cavities of sea urchins<ref>{{harvnb|James|Siikavuopio|Johansson|2018}}: "The GI [% of body weight made up by the gonad] of urchins in the wild can vary hugely and can be less than 1% or as high as 20%, whilst for cultured sea urchins GI values can be as high as 35%"</ref> and sea cucumbers, while the less voluminous crinoids, brittle stars and starfish have two gonads in each arm. While the ancestors of modern echinoderms are believed to have had one genital aperture, many organisms have multiple [[gonopore]]s through which eggs or sperm may be released.<ref>{{harvnb|Ruppert|Fox|Barnes|2004|pp=872–929}}</ref>