104-416: Ophiocanops fugiens is a living species in the brittle star family Ophiocanopidae . Though once considered to be the only one living species in this brittle star family, recent research has brought to light three specimens of Ophiocanops that differ substantially from O. fugiens. It has been regarded as the most primitive brittle star, close to Paleozoic forms, though other authors have disagreed with
208-554: A gastrula or even a blastula stage. New larvae can develop from the preoral hood (a mound like structure above the mouth), the side body wall, the postero-lateral arms, or their rear ends. Cloning is costly to the larva both in resources and in development time. Larvae undergo this process when food is plentiful or temperature conditions are optimal. Cloning may occur to make use of the tissues that are normally lost during metamorphosis. The larvae of some sand dollars clone themselves when they detect dissolved fish mucus, indicating
312-427: A yolk-feeding larva. The provision of a yolk-sac means that smaller numbers of eggs are produced, the larvae have a shorter development period and a smaller dispersal potential, but a greater chance of survival. Echinoderms are globally distributed in almost all depths, latitudes and environments in the ocean. Adults are mainly benthic , living on the seabed, whereas larvae are often pelagic , living as plankton in
416-679: A central, armored, disk-shape body that is clearly demarcated from the arms and is 3mm in diameter. Brittle Stars, including O. fugiens , have long flexible arms that they use to propel themselves through the water by moving them in a snake-like manner- hence the other common name for Brittle Stars- "Snake Stars". The arms reach lengths of 35-40mm, and are roughly 1mm across. The incredibly mobile arms are supported by an internal skeleton composed of calcium carbonate plates. These plates resemble vertebrae, and are appropriately named vertebral ossicles , in which these are controlled by O. fugiens muscular system. O. fugiens displays calcium carbonate plates on
520-464: A change in direction is necessary. Over 60 species of brittle stars are known to be bioluminescent . Most of these produce light in the green wavelengths, although a few blue-emitting species have also been discovered. Both shallow-water and deep-sea species of brittle stars are known to produce light. Presumably, this light is used to deter predators . Brittle stars live in areas from the low-tide level downwards. Six families live at least 2 m deep;
624-433: A co-ordinated way, propelled by the other four arms. During locomotion, the propelling arms can made either snake-like or rowing movements. Starfish move using their tube feet, keeping their arms almost still, including in genera like Pycnopodia where the arms are flexible. The oral surface is covered with thousands of tube feet which move out of time with each other, but not in a metachronal rhythm ; in some way, however,
728-741: A dark brown or purple in color, sometimes exhibiting spotted coloration. Ophiocanops fugiens is a species- once thought to be extinct- found in South Asian seas, with studied specimens being found in the shores of the Philippines. O. fugiens can be found in waters as shallow as the seashore floor to depths of 200 meters. O. fugiens has been found to have a mutualistic symbiotic relationship with Black Corals. O. fugiens feeds on potentially harmful microorganisms and micro-particles that inhabit coral reefs. Additionally, O. fugiens may scavenge benthic floors for detritus and plankton, or alternatively, can use
832-403: A defensive mechanism when attacked. 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
936-639: A distinguishable genital slit, also apparent on the arms. Juvenile development is lecithotrophic, meaning the larvae feed on yolk in order to develop to metamorphosis. As larvae develops, the anus is lost, and intestines are not apparent. This Ophiuroidea -related article is a stub . You can help Misplaced Pages by expanding it . Brittle star Subclass Myophiuroidea Matsumoto, 1915 Brittle stars , serpent stars , or ophiuroids (from Latin ophiurus 'brittle star'; from Ancient Greek ὄφις (óphis) 'serpent' and οὐρά (ourá) 'tail'; referring to
1040-520: A few species, the female carries a dwarf male, clinging to it with the mouth. Some brittle stars, such as the six-armed members of the family Ophiactidae , exhibit fissiparity (division through fission), with the disk splitting in half. Regrowth of both the lost part of the disk and the arms occur which yields an animal with three large arms and three small arms during the period of growth. The West Indian brittle star, Ophiocomella ophiactoides , frequently undergoes asexual reproduction by fission of
1144-474: A large stomach and a rectum with the anus at the apex of the test. Sea cucumbers are mostly detritivores , 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 . Crinoids are suspension feeders , passively catching plankton which drift into their outstretched arms. Boluses of mucus-trapped food are passed to
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#17331217605371248-620: A little in front of the midpoint. The two halves each regenerate their missing organs over a period of several months, but the missing genital organs are often very slow to develop. The larvae of some echinoderms are capable of asexual reproduction. This has long been known to occur among starfish and brittle stars, but has more recently been observed in a sea cucumber, a sand dollar and a sea urchin. This may be by autotomising parts that develop into secondary larvae, by budding , or by splitting transversely . Autotomised parts or buds may develop directly into fully formed larvae, or may pass through
1352-478: A main line of defence against potential pathogens. 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). The coelomocytes secrete antimicrobial peptides against bacteria, and have a set of lectins and complement proteins as part of an innate immune system that
1456-430: A pore (or a pair of pores in sea urchins) to the exterior, forming a podium or 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. The organisation of the water vascular system is somewhat different in ophiuroids, where
1560-462: A radial nerve which runs to the end of the limb. The nerves in each limb run through a canal at the base of the vertebral ossicles. Most ophiuroids have no eyes, or other specialised sense organs. However, they have several types of sensitive nerve endings in their epidermis, and are able to sense chemicals in the water, touch, and even the presence or absence of light. Moreover, tube feet may sense light as well as odors. These are especially found at
1664-449: A sea urchin has an 'echinopluteus' larva while a brittle star has an 'ophiopluteus' larva. A starfish has a ' bipinnaria ' larva, which develops into a multi-armed ' brachiolaria ' larva. A sea cucumber's larva is an 'auricularia' while a crinoid's is a 'vitellaria'. All these larvae are bilaterally symmetrical and have bands of cilia with which they swim; some, usually known as 'pluteus' larvae, have arms. When fully developed they settle on
1768-541: A sea urchin is overturned, it can extend its tube feet in one ambulacral area far enough to bring them within reach of the substrate and then successively attach feet from the adjoining area until it is righted. Some species bore into rock, usually by grinding away at the surface with their mouthparts. Sea cucumbers are generally sluggish animals. Many can move on the surface of the seabed or burrow through sand or mud using peristaltic movements; some have short tube feet on their under surface with which they can creep along in
1872-538: A sudden encirclement by their flexible arms. The limbs then bend under the disc to transfer the food to the jaws and mouth. Many sea urchins feed on algae, often scraping off the thin layer of algae covering the surfaces of rocks with their specialised mouthparts known as Aristotle's lantern. Other species devour smaller organisms, which they may catch with their tube feet. They may also feed on dead fish and other animal matter. Sand dollars may perform suspension feeding and feed on phytoplankton , detritus, algal pieces and
1976-459: A tip shaped like a suction pad in which a vacuum can be created by contraction of muscles. This combines with some stickiness from the secretion of mucus to provide adhesion. The tube feet contract and relax in waves which move along the adherent surface, and the animal moves slowly along. Brittle stars are the most agile of the echinoderms. Any one of the arms can form the axis of symmetry, pointing either forwards or back. The animal then moves in
2080-543: A very small number of species, the eggs are retained in the coelom where they develop viviparously , later emerging through ruptures in the body wall. In some crinoids, the embryos develop in special breeding bags, where the eggs are held until sperm released by a male happens to find them. One species of seastar , Ophidiaster granifer , reproduces asexually by parthenogenesis . In certain other asterozoans , adults reproduce asexually until they mature, then reproduce sexually. In most of these species, asexual reproduction
2184-461: A way similar to lizards which deliberately shed the distal part of their tails to confuse pursuers. Moreover, the Amphiuridae can regenerate gut and gonad fragments lost along with the arms. Discarded arms have not been shown to have the ability to regenerate. Brittle stars use their arms for locomotion. Brittle stars move fairly rapidly by wriggling their arms which are highly flexible and enable
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#17331217605372288-463: Is 89 days, so theoretically, each brittle star can produce 15 new individuals during the course of a year. Brittle stars generally sexually mature in two to three years, become full grown in three to four years, and live up to five years. Members of Euryalina , such as Gorgonocephalus , may live much longer. Ophiuroids can readily regenerate lost arms or arm segments unless all arms are lost. Ophiuroids use this ability to escape predators , in
2392-446: 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 bivalve mollusc or preventing itself from being extracted from a crevice. Similarly, sea urchins can lock their normally mobile spines upright as
2496-428: Is any animal of the phylum Echinodermata ( / ɪ ˌ k aɪ n oʊ ˈ d ɜːr m ə t ə / ), which includes starfish , brittle stars , sea urchins , sand dollars and sea cucumbers , as well as the sessile sea lilies or "stone lilies". While bilaterally symmetrical as larvae , as adults echinoderms are recognisable by their usually five-pointed radial symmetry (pentamerous symmetry), and are found on
2600-416: Is by transverse fission with the disc splitting in two. Both the lost disc area and the missing arms regrow, so an individual may have arms of varying lengths. During the period of regrowth, they have a few tiny arms and one large arm, and are thus often known as "comets". Adult sea cucumbers reproduce asexually by transverse fission. Holothuria parvula uses this method frequently, splitting into two
2704-496: Is especially common in cold water species where planktonic larvae might not be able to find sufficient food. These retained eggs are usually few in number and are supplied with large yolks to nourish the developing embryos. In starfish, the female may carry the eggs in special pouches, under her arms, under her arched body, or even in her cardiac stomach. Many brittle stars are hermaphrodites; they often brood their eggs, usually in special chambers on their oral surfaces, but sometimes in
2808-409: Is known regarding it. O. fugiens. It's incredibly small and compact central disc is proposed to have developed to lessen pressure of predation, as O. fugiens is capable of regenerating arms. The reproductive tendencies of O. fugiens are not well known. Highly developed gonads extend into the arms, and it is presumed that female O. fugiens carry fertilized eggs in the arms, and release them through
2912-426: Is limited to bending (their stems can bend) and rolling and unrolling their arms; a few species can relocate themselves on the seabed by crawling. The sea feathers are unattached and usually live in crevices, under corals or inside sponges with their arms the only visible part. Some sea feathers emerge at night and perch themselves on nearby eminences to better exploit food-bearing currents. Many species can "walk" across
3016-460: Is made up of embedded ossicles . Of all echinoderms, the Ophiuroidea may have the strongest tendency toward five-segment radial (pentaradial) symmetry . The body outline is similar to that of starfish , in that ophiuroids have five arms joined to a central body disk. However, in ophiuroids, the central body disk is sharply marked off from the arms. The disk contains all of the viscera. That is,
3120-402: 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
3224-478: Is omnivorous and feeds on algae, polychaetes and detritus. In basket stars , the arms are used to sweep food rhythmically to the mouth. Ophiopsammus maculata consumes Nothofagus pollen in the New Zealand fjords (since those trees hang over the water). Eurylina clings to coral branches to browse on the polyps . Gas exchange and excretion occur through cilia-lined sacs called bursae; each opens between
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3328-441: Is still being characterised. 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 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
3432-482: Is synchronised in some species, usually with regard to the lunar cycle. In other species, individuals may aggregate during the reproductive season, increasing the likelihood of successful fertilisation. Internal fertilisation has been observed in three species of sea star, three brittle stars and a deep-water sea cucumber. Even at abyssal depths , where no light penetrates, echinoderms often synchronise their reproductive activity. Some echinoderms brood their eggs . This
3536-755: The Agrio Formation of Neuquén Basin in the 2010s no fossil brittle star was known in the Southern Hemisphere , nor was any brittle star of Cretaceous age known. Silurian fossils from a minor mass extinction called the Mulde event shows the ancestors of modern brittle stars went though a bottleneck, where a miniaturization caused by paedomorphosis led to structural simplification of their skeletal anatomy. These traits affected their further evolution. As they began to increase in size again, so did their complexity. The first large-sized modern brittle star originated in
3640-593: The Early Ordovician . Ophiuroids can be found today in all of the major marine provinces, from the poles to the tropics. Basket stars are usually confined to the deeper parts of this range; Ophiuroids are known even from abyssal (>6,000 m) depths. However, brittle stars are also common members of reef communities, where they hide under rocks and even within other living organisms. A few ophiuroid species can even tolerate brackish water , an ability otherwise almost unknown among echinoderms. A brittle star's skeleton
3744-513: The Paleozoic era, brittle stars had open ambulacral grooves, but in modern forms, these are turned inward. In living ophiuroids, the vertebrae are linked by well-structured longitudinal muscles . Ophiuroida moves horizontally, and Euryalina species moves vertically. The latter have bigger vertebrae and smaller muscles. They are less spasmodic, but can coil their arms around objects, holding on even after death. These movement patterns are distinct to
3848-553: The crown-of-thorns starfish are long and sharp and can cause a painful puncture wound as the epithelium covering them contains a toxin. Because of their catch connective tissue, which can change rapidly from a flaccid to a rigid state, echinoderms are very difficult to dislodge from crevices. Some sea cucumbers have a cluster of cuvierian tubules which can be ejected as long sticky threads from their anus to entangle and permanently disable an attacker. Sea cucumbers occasionally defend themselves by rupturing their body wall and discharging
3952-760: The order Apodida have a single statocyst adjoining each radial nerve, and some have an eyespot at the base of each tentacle. The gonads at least periodically occupy much of the body cavities of sea urchins 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 gonopores through which eggs or sperm may be released. Many echinoderms have great powers of regeneration . Many species routinely autotomize and regenerate arms and viscera . Sea cucumbers often discharge parts of their internal organs if they perceive themselves to be threatened, regenerating them over
4056-426: The sea-lily Comaster schlegelii has two hundred. 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
4160-465: The water vascular system end in tube feet . The water vascular system generally has one madreporite. Others, such as certain Euryalina, have one per arm on the aboral surface. Still other forms have no madreporite at all. Suckers and ampullae are absent from the tube feet. The nervous system consists of a main nerve ring which runs around the central disk. At the base of each arm, the ring attaches to
4264-598: The Asteroidea ( starfish , with some 1,745 species), Ophiuroidea ( brittle stars , with around 2,300 species), Echinoidea ( sea urchins and sand dollars , with some 900 species), Holothuroidea ( sea cucumbers , with about 1,430 species), and Crinoidea ( feather stars and sea lilies , with around 580 species). Echinoderms evolved from animals with bilateral symmetry . Although adult echinoderms possess pentaradial symmetry, their larvae are ciliated , free-swimming organisms with bilateral symmetry. Later, during metamorphosis,
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4368-548: The Early Carboniferous. Brittle stars are not used as food, though they are not toxic, because of their strong skeleton. Even if some species have blunt spines, no brittlestar is known to be dangerous, nor venomous. There is no harm evidence towards humans, and even with their predators, brittlestars' only means of defense is escaping or discarding an arm. Brittle stars are a moderately popular invertebrate in fishkeeping . They can easily thrive in marine tanks; in fact,
4472-730: The ITIS rules should result in attributing "Klein, 1778" due to a 2nd edition of his work published by Leske in that year. While Echinodermata has been in common use since the mid-1800s, several other names had been proposed. Notably, F. A. Bather called the phylum "Echinoderma" (apparently after Latreille , 1825 ) in his 1900 treatise on the phylum, but this name now refers to a fungus . There are about 7,600 extant species of echinoderm as well as about 13,000 extinct species. All echinoderms are marine , but they are found in habitats ranging from shallow intertidal areas to abyssal depths. Five extant classes of echinoderms are generally recognized:
4576-421: The animal. The madreporite is linked to a slender duct, the stone canal, which extends to a ring canal that encircles the mouth or 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
4680-445: The animals to make either snake-like or rowing movements. However, they tend to attach themselves to the sea floor or to sponges or cnidarians, such as coral. They move as if they were bilaterally symmetrical, with an arbitrary leg selected as the symmetry axis and the other four used in propulsion. The axial leg may be facing or trailing the direction of motion, and due to the radially symmetrical nervous system, can be changed whenever
4784-597: 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. 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 molluscs . Brittle stars, which have varying diets, have a blind gut with no intestine or anus; they expel food waste through their mouth. Sea urchins are herbivores and use their specialised mouthparts to graze, tear and chew their food, mainly algae . They have an oesophagus,
4888-463: The arm bases on the underside of the disk. Typically ten bursae are found, and each fits between two stomach digestive pouches. Water flows through the bursae by means of cilia or muscular contraction. Oxygen is transported through the body by the hemal system , a series of sinuses and vessels distinct from the water vascular system. The bursae are probably also the main organs of excretion, with phagocytic "coelomocytes" collecting waste products in
4992-537: 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 . 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. Although individual ossicles are robust and fossilize readily, complete skeletons of starfish, brittle stars and crinoids are rare in
5096-411: The arms. The stomach wall contains glandular hepatic cells. Ophiuroids are generally scavengers or detritivores . Small organic particles are moved into the mouth by the tube feet. Ophiuroids may also prey on small crustaceans or worms. Basket stars in particular may be capable of suspension feeding, using the mucus coating on their arms to trap plankton and bacteria. They extend one arm out and use
5200-485: The bacterial layer surrounding grains of sand. Sea cucumbers are often mobile deposit or suspension feeders, using their buccal podia to actively capture food and then stuffing the particles individually into their buccal cavities. Others ingest large quantities of sediment, absorb the organic matter and pass the indigestible mineral particles through their guts. In this way they disturb and process large volumes of substrate, often leaving characteristic ridges of sediment on
5304-420: The biotic desert of the deep sea , as well as shallower oceans . Most echinoderms are able to reproduce asexually and regenerate tissue, organs and limbs; in some cases, they can undergo complete regeneration from a single limb. Geologically, the value of echinoderms is in their ossified dermal endoskeletons , which are major contributors to many limestone formations and can provide valuable clues as to
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#17331217605375408-438: The body and arms, presumably for defensive measures, and the arms bear delicate spines. The arms lack dorsal and ventral shields; the madreporite is on edge of central disc. The digestive glands extend into proximal portions of arms. It has no bursae for gonads and gas exchange/excretion. O. fugiens demonstrates a small, triangular plated mouth, featuring a set of six conically arranged teeth. O. fugiens has been reported to be
5512-417: The body cavity and then migrating to the bursae for expulsion from the body. Like all echinoderms, the Ophiuroidea possess a skeleton of calcium carbonate in the form of calcite . In ophiuroids, the calcite ossicles are fused to form armor plates which are known collectively as the test . The plates are covered by the epidermis , which consists of a smooth syncytium . In most species, the joints between
5616-448: The body surface. This means that a starfish body can more-or-less be considered to consist only of a head. Echinoderms have a mesodermal skeleton in the dermis, composed of calcite -based plates known as ossicles . If solid, these would form a heavy skeleton, so they have a sponge-like porous structure known as stereom. Ossicles may be fused together, as in the test of sea urchins, or may articulate to form flexible joints as in
5720-489: The brittle stars, six-armed species such as Ophiothela danae , Ophiactis savignyi , and Ophionotus hexactis exist, and Ophiacantha vivipara often has more than six. 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. Many crinoids and some seastars are symmetrical in multiples of the basic five; starfish such as Labidiaster annulatus possess up to fifty arms, while
5824-417: The course of several months. Sea urchins constantly replace spines lost through damage, while sea stars and sea lilies readily lose and regenerate their arms. In most cases, a single severed arm cannot grow into a new starfish in the absence of at least part of the disc. However, in a few species a single arm can survive and develop into a complete individual, and arms are sometimes intentionally detached for
5928-406: The disk with subsequent regeneration of the arms. In both summer and winter, large numbers of individuals with three long arms and three short arms can be found. Other individuals have half a disk and only three arms. A study of the age range of the population indicates little recruitment and fission is the primary means of reproduction in this species. In this species, fission appears to start with
6032-561: The disk, and open into pouches between the arms, called genital bursae. Fertilization is external in most species, with the gametes being shed into the surrounding water through the bursal sacs. An exception is the Ophiocanopidae, in which the gonads do not open into bursae and are instead paired in a chain along the basal arm joints. Many species brood developing larvae in the bursae, effectively giving birth to live young. A few, such as Amphipholus squamata , are truly viviparous , with
6136-399: The ecological roles of adults are the grazing of sea urchins, the sediment processing of heart urchins, and the suspension and deposit feeding of crinoids and sea cucumbers. Some sea urchins can bore into solid rock, destabilising rock faces and releasing nutrients into the ocean. Coral reefs are also bored into in this way, but the rate of accretion of carbonate material is often greater than
6240-471: The embryo receiving nourishment from the mother through the wall of the bursa. However, some species do not brood their young, and instead have a free-swimming larval stage. Referred to as an ophiopluteus , these larvae have four pairs of rigid arms lined with cilia . They develop directly into an adult, without the attachment stage found in most starfish larvae. The number of species exhibiting ophiopluteus larvae are fewer than those that directly develop. In
6344-428: The ends of their arms, detecting light and retreating into crevices. The mouth is rimmed with five jaws, and serves as an anus ( egestion ) as well as a mouth ( ingestion ). Behind the jaws is a short esophagus and a stomach cavity which occupies much of the dorsal half of the disk. Digestion occurs within 10 pouches or infolds of the stomach, which are essentially ceca , but unlike in sea stars, almost never extend into
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#17331217605376448-451: The epithelium and have simple eyespots and touch-sensitive tentacle-like tube feet at the tips of their arms. Sea urchins have no particular sense organs but do have statocysts 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
6552-447: The form of ammonia , diffuses out through the respiratory surfaces. The coelomic fluid contains the coelomocytes , or immune cells. There are several types of immune cells, which vary among classes and species. All classes possess a type of 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
6656-441: 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, paleontologists can observe distinctive cleavage patterns and sometimes even the intricate internal and external structure of the test. The epidermis contains pigment cells that provide
6760-421: The genera Ophiura , Amphiophiura , and Ophiacantha range below 4 m. Shallow species live among sponges, stones, or coral, or under the sand or mud, with only their arms protruding. Two of the best-known shallow species are the green brittle star ( Ophioderma brevispina ), found from Massachusetts to Brazil, and the common European brittle star ( Ophiothrix fragilis ). Deep-water species tend to live in or on
6864-482: The geological environment. They were the most used species in regenerative research in the 19th and 20th centuries. Further, some scientists hold that the radiation of echinoderms was responsible for the Mesozoic Marine Revolution . The name echinoderm is from Ancient Greek ἐχῖνος ( ekhînos ) 'hedgehog' and δέρμα ( dérma ) 'skin'. The name Echinodermata
6968-416: The great quantity of eggs and larva that they produce form part of the zooplankton , consumed by many marine creatures. Crinoids, on the other hand, are relatively free from predation. Antipredator defences include the presence of spines, toxins (inherent or delivered through the tube feet), and the discharge of sticky entangling threads by sea cucumbers. Although most echinoderm spines are blunt, those of
7072-399: The gut and internal organs. Starfish and brittle stars may undergo autotomy when attacked, detaching an arm; this may distract the predator for long enough for the animal to escape. Some starfish species can swim away from danger. Echinoderms are numerous invertebrates whose adults play an important role in benthic ecosystems , while the larvae are a major component of the plankton. Among
7176-446: The internal organs of digestion and reproduction never enter the arms, as they do in the Asteroidea. The underside of the disk contains the mouth, which has five toothed jaws formed from skeletal plates. The madreporite is usually located within one of the jaw plates, and not on the upper side of the animal as it is in starfish. The ophiuroid coelom is strongly reduced, particularly in comparison to other echinoderms. The vessels of
7280-512: 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 pentaradially symmetric fashion, in which the body is arranged in five parts around a central axis. Within the Asterozoa , there can be 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
7384-452: The madreporite may be on the oral surface and the podia lack suckers. 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. The arrangement in crinoids is similar to that in asteroids, but
7488-774: The majority of starfish are active hunters. Crinoids catch food particles using the tube feet on their outspread pinnules, move them into the ambulacral grooves, wrap them in mucus, and convey them to the mouth using the cilia lining the grooves. The exact dietary requirements of crinoids have been little researched, but in the laboratory, they can be fed with diatoms. Basket stars are suspension feeders, raising their branched arms to collect zooplankton , while other brittle stars use several methods of feeding. Some are suspension feeders, securing food particles with mucus strands, spines or tube feet on their raised arms. Others are scavengers and detritus feeders. Others again are voracious carnivores and able to lasso their waterborne prey with
7592-478: The manner of a starfish. Some species drag themselves along using their buccal tentacles, while others manage to swim with peristaltic movements or rhythmic flexing. Many live in cracks, hollows and burrows and hardly move at all. Some deep-water species are pelagic and can float in the water with webbed papillae forming sails or fins. The majority of crinoids are motile, but sea lilies are sessile and attached to hard substrates by stalks. Movement in most sea lilies
7696-473: The micro brittle star is a common "hitchhiker" that will propagate and become common in almost any saltwater tank, if one happens to come along on some live rock . Larger brittle stars are popular because, unlike Asteroidea, they are not generally seen as a threat to coral, and are also faster-moving and more active than their more archetypical cousins. Echinoderm See taxonomy An echinoderm ( / ɪ ˈ k aɪ n ə ˌ d ɜːr m , ˈ ɛ k ə -/ )
7800-536: The mouth, which is linked to the anus by a loop consisting of a short oesophagus and longer intestine. The coelomic cavities of echinoderms are complex. Aside from the water vascular system, echinoderms have a haemal coelom , a peri visceral coelom, a gonadal coelom and often also a perihaemal coelom. During development, echinoderm coelom is divided into the metacoel, mesocoel and protocoel (also called somatocoel, hydrocoel and axocoel, respectively). The water vascular system, haemal system and perihaemal system form
7904-536: The movement and remodelling of existing tissues to replace lost parts. Direct transdifferentiation of one type of tissue to another during tissue replacement is also observed. Echinoderms become sexually mature after approximately two to three years, depending on the species and the environmental conditions. Almost all species have separate male and female sexes , though some are hermaphroditic . The eggs and sperm cells are typically released into open water, where fertilisation takes place. The release of sperm and eggs
8008-459: The often vivid colours of echinoderms, which include deep red, stripes of black and white, and intense purple. 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. One characteristic of most echinoderms
8112-526: The open ocean. Some holothuroid adults such as Pelagothuria are however pelagic. Some crinoids are pseudo-planktonic, attaching themselves to floating logs and debris, although this behaviour was exercised most extensively in the Paleozoic, before competition from organisms such as barnacles restricted the extent of the behaviour. Echinoderms primarily use their tube feet to move about, though some sea urchins also use their spines. The tube feet typically have
8216-617: The ossicles and superficial plates allow the arm to bend to the side, but cannot bend upwards. However, in the basket stars , the arms are flexible in all directions. Both the Ophiurida and Euryalida (the basket stars) have five long, slender, flexible, whip-like arms, up to 60 cm in length. They are supported by an internal skeleton of calcium carbonate plates referred to as vertebral ossicles. These " vertebrae " articulate by means of ball-and-socket joints , and are controlled by muscles. They are essentially fused plates which correspond to
8320-551: The other four as anchors. Brittle stars will eat small suspended organisms if available. In large, crowded areas, brittle stars eat suspended matter from prevailing seafloor currents. Many species in the family Ophiuridae are carnivorous. Ophiura Linnaeus hunts epibenthic animals and the Antarctic Ophiosparte gigas is an active predator. Ophiura albida Forbes and Ophiura sarsii Lütken eat both infaunal prey, carrion and seafloor organic matter, and Ophionereis reticulata
8424-424: The ovary or coelom. In these starfish and brittle stars, development is usually direct to the adult form, without passing through a bilateral larval stage. A few sea urchins and one species of sand dollar carry their eggs in cavities, or near their anus, holding them in place with their spines. Some sea cucumbers use their buccal tentacles to transfer their eggs to their underside or back, where they are retained. In
8528-458: The parallel ambulacral plates in sea stars and five Paleozoic families of ophiuroids. In modern forms, the vertebrae occur along the median of the arm. The ossicles are surrounded by a relatively thin ring of soft tissue, and then by four series of jointed plates, one each on the upper, lower, and lateral surfaces of the arm. The two lateral plates often have a number of elongated spines projecting outwards; these help to provide traction against
8632-425: The presence of predators. Asexual reproduction produces many smaller larvae that escape better from planktivorous fish, implying that the mechanism may be an anti-predator adaptation. Development begins with a bilaterally symmetrical embryo, with a coeloblastula developing first. Gastrulation marks the opening of the "second mouth" that places echinoderms within the deuterostomes, and the mesoderm, which will host
8736-399: The propellant motion of their arms to capture surrounding particles to feed on. The arms serve further purpose as well. O. fugiens lacks eyes, but receives and responds to changes in light via photoreceptive tissues in their appendages. There are no reports of sexual dimorphism within the species. The evolutionary development of O. fugiens has been largely classified as an enigma, and little
8840-459: The purpose of asexual reproduction . During periods when they have lost their digestive tracts, sea cucumbers live off stored nutrients and absorb dissolved organic matter directly from the water. The regeneration of lost parts involves both epimorphosis and morphallaxis . In epimorphosis stem cells—either from a reserve pool or those produced by dedifferentiation —form a blastema and generate new tissues. Morphallactic regeneration involves
8944-507: The sea bed at every ocean depth from the intertidal zone to the abyssal zone . The phylum contains about 7,600 living species , making it the second-largest group of deuterostomes after the chordates , as well as the largest marine-only phylum. The first definitive echinoderms appeared near the start of the Cambrian . The echinoderms are important both ecologically and geologically. Ecologically, there are few other groupings so abundant in
9048-589: The sea floor or adhere to coral, urchins, or xenophyophores . The most widespread species is the long-armed brittle star ( Amphipholis squamata ), a grayish or bluish, strongly luminescent species. The main parasite to enter the digestive tract or genitals are protozoans. Crustaceans, nematodes, trematodes, and polychaete annelids also serve as parasites. Algal parasites such as Coccomyxa ophiurae cause spinal malformation. Unlike in sea stars and sea urchins, annelids are not typical parasites. Between 2,064 and 2,122 species of brittle stars are currently known, but
9152-449: The seabed to undergo metamorphosis, and the larval arms and gut degenerate. The left-hand side of the larva develops into the oral surface of the juvenile, while the right side becomes the aboral surface. At this stage the pentaradial symmetry develops. A plankton-eating larva, living and feeding in the water column, is considered to be the ancestral larval type for echinoderms, but in extant echinoderms, some 68% of species develop using
9256-470: The seabed, raising their body with the help of their arms, or swim using their arms. Most species of sea feather, however, are largely sedentary, seldom moving far from their chosen place of concealment. The modes of feeding vary greatly between the different echinoderm taxa. Crinoids and some brittle stars tend to be passive filter-feeders, enmeshing suspended particles from passing water. Most sea urchins are grazers; sea cucumbers are deposit feeders; and
9360-441: The seabed. Some sea cucumbers live infaunally in burrows, anterior-end down and anus on the surface, swallowing sediment and passing it through their gut. Other burrowers live anterior-end up and wait for detritus to fall into the entrances of the burrows or rake in debris from the surface nearby with their buccal podia. Nearly all starfish are detritus feeders or carnivores, though a few are suspension feeders. Small fish landing on
9464-552: The sediment leaving only their arms in the stream to capture the plankton). There are also 344 species in the family of Ophiuridae . List of families according to the World Register of Marine Species , following O'Hara 2017: The first known brittle stars date from Early Ordovician . Study of past distribution and evolution of brittle stars has been hampered by the tendency of dead brittle stars to disarticulate and scatter, providing poor brittle star fossils. Until discoveries in
9568-609: The serpent-like arms of the brittle star) are echinoderms in the class Ophiuroidea , closely related to starfish . They crawl across the sea floor using their flexible arms for locomotion. The ophiuroids generally have five long, slender, whip-like arms which may reach up to 60 cm (24 in) in length on the largest specimens. The Ophiuroidea contain two large clades , Ophiurida (brittle stars) and Euryalida (basket stars). Over 2,000 species of brittle stars live today. More than 1,200 of these species are found in deep waters, greater than 200 m deep. The ophiuroids diverged in
9672-416: The skeleton, migrates inwards. The secondary body cavity, the coelom, forms by the partitioning of three body cavities. The larvae are often planktonic , but in some species the eggs are retained inside the female, while in some the female broods the larvae. The larvae pass through several stages, which have specific names derived from the taxonomic names of the adults or from their appearance. For example,
9776-649: The soft body parts. As the adductor muscle of the bivalve relaxes, more stomach is inserted and when digestion is complete, the stomach is returned to its usual position in the starfish with its now liquefied bivalve meal inside it. Other starfish evert the stomach to feed on sponges, sea anemones, corals, detritus and algal films. Despite their low nutrition value and the abundance of indigestible calcite, echinoderms are preyed upon by many organisms, including bony fish , sharks , eider ducks , gulls , crabs , gastropod molluscs , other echinoderms, sea otters , Arctic foxes and humans. Larger starfish prey on smaller ones;
9880-427: The softening of one side of the disk and the initiation of a furrow. This deepens and widens until it extends across the disk and the animal splits in two. New arms begin to grow before the fission is complete, thus minimizing the time between possible successive divisions. The plane of fission varies so that some newly formed individuals have existing arms of different lengths. The time period between successive divisions
9984-450: The substrate while the animal is moving. The spines, in ophiuroids, compose a rigid border to the arm edges, whereas in euryalids they are transformed into downward-facing clubs or hooklets. Euryalids are similar to ophiurids, if larger, but their arms are forked and branched. Ophiuroid podia generally function as sensory organs. They are not usually used for feeding, as in Asteroidea . In
10088-453: The taxa, separating them. Ophiuroida moves quickly when disturbed. One arm presses ahead, whereas the other four act as two pairs of opposite levers, thrusting the body in a series of rapid jerks. Although adults do not use their tube feet for locomotion, very young stages use them as stilts and even serve as an adhesive structure. The sexes are separate in most species, though a few are hermaphroditic or protandric . The gonads are located in
10192-448: The total number of modern species may be over 3,000. This makes brittle stars the most abundant group of current echinoderms (before sea stars). Around 270 genera are known, these are distributed in 16 families, which makes them at the same time a relatively poorly diversified group structurally, compared with the other echinoderms. For example, 467 species belong to the sole family of Amphiuridae (frail brittle stars which live buried in
10296-411: The tube feet are coordinated, as the animal glides steadily along. Some burrowing starfish have points rather than suckers on their tube feet and they are able to "glide" across the seabed at a faster rate. Sea urchins use their tube feet to move around in a similar way to starfish. Some also use their articulated spines to push or lever themselves along or lift their oral surfaces off the substrate. If
10400-430: 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. 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
10504-403: The tubular coelomic system. 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. Haemal and perihaemal systems are derived from the original coelom, forming an open and reduced circulatory system. This usually consists of a central ring and five radial vessels. There
10608-447: The upper surface may be captured by pedicilaria and dead animal matter may be scavenged but the main prey items are living invertebrates, mostly bivalve molluscs. To feed on one of these, the starfish moves over it, attaches its tube feet and exerts pressure on the valves by arching its back. When a small gap between the valves is formed, the starfish inserts part of its stomach into the prey, excretes digestive enzymes and slowly liquefies
10712-401: The view. Classification of O. fugiens is highly argued. Ophiocanops is usually placed in the order Oegophiurida (suborder Zeugophiurina ) or regarded as a genus incertae sedis or even given its own subclass Oegophiuridea. Some recent data suggest its relationship to the extant family Ophiomyxidae (it would thus belong to the order Phrynophiurida ). Ophiocanops fugiens displays
10816-523: Was originated by Jacob Theodor Klein in 1734, but only in reference to echinoids . It was expanded to the phylum level by Jean Guillaume Bruguière , first informally in 1789 and then in formal Latin in 1791. In 1955, Libbie Hyman attributed the name to "Bruguière, 1791 [ex Klein, 1734]." This attribution has become common and is listed by the Integrated Taxonomic Information System (ITIS), although some workers believe that
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