In geometry , a star polygon is a type of non- convex polygon . Regular star polygons have been studied in depth; while star polygons in general appear not to have been formally defined, certain notable ones can arise through truncation operations on regular simple or star polygons.
91-452: † Calliasterellidae † Trichasteropsida Starfish or sea stars are star-shaped echinoderms belonging to the class Asteroidea ( / ˌ æ s t ə ˈ r ɔɪ d i ə / ). Common usage frequently finds these names being also applied to ophiuroids , which are correctly referred to as brittle stars or basket stars. Starfish are also known as asteroids due to being in the class Asteroidea. About 1,900 species of starfish live on
182-489: A numeral prefix , such as penta- , with the Greek suffix -gram (in this case generating the word pentagram ). The prefix is normally a Greek cardinal , but synonyms using other prefixes exist. For example, a nine-pointed polygon or enneagram is also known as a nonagram , using the ordinal nona from Latin . The -gram suffix derives from γραμμή ( grammḗ ), meaning a line. The name star polygon reflects
273-605: A tessellated manner and form the main covering of the aboral surface. Some are specialised structures such as the madreporite (the entrance to the water vascular system), pedicellariae and paxillae . Pedicellariae are compound ossicles with forceps-like jaws. They remove debris from the body surface and wave around on flexible stalks in response to physical or chemical stimuli while continually making biting movements. They often form clusters surrounding spines. Paxillae are umbrella-like structures found on starfish that live buried in sediment. The edges of adjacent paxillae meet to form
364-567: A wasting condition caused by bacteria in the genus Vibrio ; however, a more widespread wasting disease , causing mass mortalities among starfish, appears sporadically. A paper published in November 2014 revealed the most likely cause of this disease to be a densovirus the authors named sea star-associated densovirus (SSaDV). The protozoan Orchitophrya stellarum is known to infect the gonads of starfish and damage tissue. Starfish are vulnerable to high temperatures. Experiments have shown that
455-489: A 1971 study of Stichaster australis on the intertidal coast of the South Island of New Zealand . S. australis was found to have removed most of a batch of transplanted mussels within two or three months of their placement, while in an area from which S. australis had been removed, the mussels increased in number dramatically, overwhelming the area and threatening biodiversity . The feeding activity of
546-469: A cascading effect on the whole benthic community and reef-feeding fish. Asterias amurensis is one of a few echinoderm invasive species . Its larvae likely arrived in Tasmania from central Japan via water discharged from ships in the 1980s. The species has since grown in numbers to the point where they threaten commercially important bivalve populations. As such, they are considered pests, and are on
637-409: A central disc and multiple radiating arms. The subphylum includes the two classes of Asteroidea, the starfish, and Ophiuroidea , the brittle stars and basket stars. Asteroids have broad-based arms with skeletal support provided by calcareous plates in the body wall while ophiuroids have clearly demarcated slender arms strengthened by paired fused ossicles forming jointed "vertebrae". The starfish are
728-510: A degenerate polygon will result with coinciding vertices and edges. For example, {6/2} will appear as a triangle, but can be labeled with two sets of vertices: 1-3 and 4-6. This should be seen not as two overlapping triangles, but as a double-winding single unicursal hexagon. Alternatively, a regular star polygon can also be obtained as a sequence of stellations of a convex regular core polygon. Constructions based on stellation also allow regular polygonal compounds to be obtained in cases where
819-450: A difference when retrograde polygons are incorporated in higher-dimensional polytopes. For example, an antiprism formed from a prograde pentagram {5/2} results in a pentagrammic antiprism ; the analogous construction from a retrograde "crossed pentagram" {5/3} results in a pentagrammic crossed-antiprism . Another example is the tetrahemihexahedron , which can be seen as a "crossed triangle" {3/2} cuploid . If p and q are not coprime,
910-432: A false cuticle with a water cavity beneath in which the madreporite and delicate gill structures are protected. All the ossicles, including those projecting externally, are covered by the epidermal layer. Several groups of starfish, including Valvatida and Forcipulatida , possess pedicellariae . In Forcipulatida, such as Asterias and Pisaster , they occur in pompom -like tufts at the base of each spine, whereas in
1001-494: A free-living juvenile starfish about 1 mm (0.04 in) in diameter. Starfish of the order Paxillosida have no brachiolaria stage, with the bipinnaria larvae settling on the seabed and developing directly into juveniles. Some species of starfish in the three families Asterinidae, Asteriidae and Solasteridae are able to reproduce asexually as adults either by fission of their central discs or by autotomy of one or more of their arms. Which of these processes occurs depends on
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#17327870178111092-543: A large and diverse class with over 1,900 living species. There are seven extant orders, Brisingida , Forcipulatida , Notomyotida , Paxillosida , Spinulosida , Valvatida and Velatida and two extinct ones, Calliasterellidae and Trichasteropsida . Living asteroids, the Neoasteroidea, are morphologically distinct from their forerunners in the Paleozoic. The taxonomy of the group is relatively stable but there
1183-442: A large female can split in half and the resulting offspring are males. When these grow large enough they change back into females. Each starfish arm contains two gonads that release gametes through openings called gonoducts, located on the central disc between the arms. Fertilization is generally external but in a few species, internal fertilization takes place. In most species, the buoyant eggs and sperm are simply released into
1274-422: A larva senses that food is plentiful, it takes the path of asexual reproduction rather than normal development. Though this costs it time and energy and delays maturity, it allows a single larva to give rise to multiple adults when the conditions are appropriate. Some species of starfish have the ability to regenerate lost arms and can regrow an entire new limb given time. A few can regrow a complete new disc from
1365-541: A male and female may come together and form a pair. This behaviour is called pseudocopulation and the male climbs on top, placing his arms between those of the female. When she releases eggs into the water, he is induced to spawn. Starfish may use environmental signals to coordinate the time of spawning (day length to indicate the correct time of the year, dawn or dusk to indicate the correct time of day), and chemical signals to indicate their readiness to breed. In some species, mature females produce chemicals to attract sperm in
1456-542: A maximum recorded lifespan of 34 years. The average lifespan of a starfish is 35 years, and larger starfish species typically live longer than their smaller counterparts. Echinoderms, including starfish, maintain a delicate internal electrolyte balance that is in equilibrium with sea water, making it impossible for them to live in a freshwater habitat. Starfish species inhabit all of the world's oceans. Habitats range from tropical coral reefs , rocky shores, tidal pools , mud, and sand to kelp forests , seagrass meadows and
1547-475: A means of defense. The Asteroidea occupy several significant ecological roles . Starfish, such as the ochre sea star ( Pisaster ochraceus ) and the reef sea star ( Stichaster australis ), have become widely known as examples of the keystone species concept in ecology. The tropical crown-of-thorns starfish ( Acanthaster planci ) is a voracious predator of coral throughout the Indo-Pacific region, and
1638-430: A part in literature, legend, design and popular culture. They are sometimes collected as curios , used in design or as logos, and in some cultures, despite possible toxicity, they are eaten. Most starfish have five arms that radiate from a central disc, but the number varies with the group. Some species have six or seven arms and others have 10–15 arms. The Antarctic Labidiaster annulatus can have over fifty. Mapping
1729-411: A radical rearrangement of tissues. The left side of the larval body becomes the oral surface of the juvenile and the right side the aboral surface. Part of the gut is retained, but the mouth and anus move to new positions. Some of the body cavities degenerate but others become the water vascular system and the visceral coelom. The starfish is now pentaradially symmetrical. It casts off its stalk and becomes
1820-589: A role in the distribution and abundance of organisms such as fish, crabs and sea urchins that feed on the sediment. Starfish sometimes have negative effects on ecosystems. Outbreaks of crown-of-thorns starfish have caused damage to coral reefs in Northeast Australia and French Polynesia . A study in Polynesia found that coral cover declined drastically with the arrival of migratory starfish in 2006, dropping from 50% to under 5% in three years. This had
1911-412: A single arm, while others need at least part of the central disc to be attached to the detached part. Regrowth can take several months or years, and starfish are vulnerable to infections during the early stages after the loss of an arm. A separated limb lives off stored nutrients until it regrows a disc and mouth and is able to feed again. Other than fragmentation carried out for the purpose of reproduction,
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#17327870178112002-456: A small number of alkaloids have also been identified. The functions of these chemicals in the starfish have not been fully investigated but most have roles in defence and communication. Some are feeding deterrents used by the starfish to discourage predation. Others are antifoulants and supplement the pedicellariae to prevent other organisms from settling on the starfish's aboral surface. Some are alarm pheromones and escape-eliciting chemicals,
2093-503: A starfish varies considerably between species, generally being longer in larger forms and in those with planktonic larvae. For example, Leptasterias hexactis broods a small number of large-yolked eggs. It has an adult weight of 20 g (0.7 oz), reaches sexual maturity in two years and lives for about ten years. Pisaster ochraceus releases a large number of eggs into the sea each year and has an adult weight of up to 800 g (28 oz). It reaches maturity in five years and has
2184-599: A tessellation pattern. In his 1619 work Harmonices Mundi , among periodic tilings, Johannes Kepler includes nonperiodic tilings, like that with three regular pentagons and one regular star pentagon fitting around certain vertices, 5.5.5.5/2, and related to modern Penrose tilings . The interior of a star polygon may be treated in different ways. Three such treatments are illustrated for a pentagram. Branko Grünbaum and Geoffrey Shephard consider two of them, as regular star n -gons and as isotoxal concave simple 2 n -gons. [REDACTED] These three treatments are: When
2275-536: A thin cuticle, an epidermis consisting of a single layer of cells, a thick dermis formed of connective tissue and a thin coelomic myoepithelial layer, which provides the longitudinal and circular musculature. The dermis contains an endoskeleton of calcium carbonate components known as ossicles. These are honeycombed structures composed of calcite microcrystals arranged in a lattice. They vary in form, with some bearing external granules, tubercles and spines, but most are tabular plates that fit neatly together in
2366-454: A tough peristomial membrane and closed with a sphincter . The mouth opens through a short oesophagus into a stomach divided by a constriction into a larger, eversible cardiac portion and a smaller pyloric portion. The cardiac stomach is glandular and pouched, and is supported by ligaments attached to ossicles in the arms so it can be pulled back into position after it has been everted. The pyloric stomach has two extensions into each arm:
2457-444: Is apparent when they spawn . Some species are simultaneous hermaphrodites , producing eggs and sperm at the same time, and in a few of these the same gonad, called an ovotestis , produces both eggs and sperm. Other starfish are sequential hermaphrodites . Protandrous individuals of species like Asterina gibbosa start life as males before changing sex into females as they grow older. In some species such as Nepanthia belcheri ,
2548-440: Is at the apex of a vertical channel (the axial vessel) that connects the three rings. At the base of each arm are paired gonads ; a lateral vessel extends from the genital ring past the gonads to the tip of the arm. This vessel has a blind end and there is no continuous circulation of the fluid within it. This liquid does not contain a pigment and has little or no respiratory function but is probably used to transport nutrients around
2639-618: Is covered with overlapping plates. Many species are brightly coloured in various shades of red or orange, while others are blue, grey or brown. Starfish have tube feet operated by a hydraulic system and a mouth at the centre of the oral or lower surface. They are opportunistic feeders and are mostly predators on benthic invertebrates. Several species have specialized feeding behaviours including eversion of their stomachs and suspension feeding . They have complex life cycles and can reproduce both sexually and asexually . Most can regenerate damaged parts or lost arms and they can shed arms as
2730-750: Is ongoing debate about the status of the Paxillosida , and the deep-water sea daisies, though clearly Asteroidea and currently included in Velatida , do not fit easily in any accepted lineage. Phylogenetic data suggests that they may be a sister group , the Concentricycloidea, to the Neoasteroidea, or that the Velatida themselves may be a sister group. Extinct groups within the Asteroidea include: Starfish are deuterostome animals, like
2821-417: Is passed into their pyloric stomachs and caeca where digestion continues and absorption ensues. In more advanced species of starfish, the cardiac stomach can be everted from the organism's body to engulf and digest food. When the prey is a clam or other bivalve , the starfish pulls with its tube feet to separate the two valves slightly, and inserts a small section of its stomach, which releases enzymes to digest
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2912-399: Is replaced with radial symmetry , typically pentameric . Adult echinoderms are characterized by having a water vascular system with external tube feet and a calcareous endoskeleton consisting of ossicles connected by a mesh of collagen fibres. Starfish are included in the subphylum Asterozoa , the characteristics of which include a flattened, star-shaped body as adults consisting of
3003-414: Is transferred from these to the coelomic fluid , which acts as the transport medium for gasses. Oxygen dissolved in the water is distributed through the body mainly by the fluid in the main body cavity; the circulatory system may also play a minor role. The gut of a starfish occupies most of the disc and extends into the arms. The mouth is located in the centre of the oral surface, where it is surrounded by
3094-548: The Goniasteridae , such as Hippasteria phrygiana , the pedicellariae are scattered over the body surface. Some are thought to assist in defence, while others aid in feeding or in the removal of organisms attempting to settle on the starfish's surface. Some species like Labidiaster annulatus , Rathbunaster californicus and Novodinia antillensis use their large pedicellariae to capture small fish and crustaceans. There may also be papulae , thin-walled protrusions of
3185-616: The Northern Pacific seastar is on the list of the World's 100 Worst Invasive Alien Species . The fossil record for starfish is ancient, dating back to the Ordovician around 450 million years ago, but it is rather sparse, as starfish tend to disintegrate after death. Only the ossicles and spines of the animal are likely to be preserved, making remains hard to locate. With their appealing symmetrical shape, starfish have played
3276-481: The ambulacral groove in each arm. There are short lateral canals branching off alternately to either side of the radial canal, each ending in an ampulla. These bulb-shaped organs are joined to tube feet (podia) on the exterior of the animal by short linking canals that pass through ossicles in the ambulacral groove. There are usually two rows of tube feet but in some species, the lateral canals are alternately long and short and there appear to be four rows. The interior of
3367-419: The basement membrane but generally beneath the luminal cells. These may be positive for alpha smooth muscle actin and can contract and expel the secretions of exocrine glands . They are found in the sweat glands , mammary glands , lacrimal glands , and salivary glands . Myoepithelial cells in these cases constitute the basal cell layer of an epithelium that harbors the epithelial progenitor . In
3458-421: The chordates . A 2014 analysis of 219 genes from all classes of echinoderms gives the following phylogenetic tree . The times at which the clades diverged are shown under the labels in millions of years ago (mya). Xenacoelomorpha [REDACTED] Chordata [REDACTED] Star polygon Branko Grünbaum identified two primary usages of this terminology by Johannes Kepler , one corresponding to
3549-607: The fossil record in the Cambrian . The first known asterozoans were the Somasteroidea , which exhibit characteristics of both groups. Starfish are infrequently found as fossils, possibly because their hard skeletal components separate as the animal decays. Despite this, there are a few places where accumulations of complete skeletal structures occur, fossilized in place in Lagerstätten – so-called "starfish beds". By
3640-575: The omnivorous starfish Oreaster reticulatus on sandy and seagrass bottoms in the Virgin Islands appears to regulate the diversity, distribution and abundance of microorganisms. These starfish engulf piles of sediment removing the surface films and algae adhering to the particles. Organisms that dislike this disturbance are replaced by others better able to rapidly recolonise "clean" sediment. In addition, foraging by these migratory starfish creates diverse patches of organic matter, which may play
3731-408: The pH is lowered. Researchers found that when P. ochraceus were exposed to 21 °C (70 °F) and 770 ppm carbon dioxide (beyond rises expected in the next century), they were relatively unaffected. Their survival is likely due to the nodular nature of their skeletons, which are able to compensate for a shortage of carbonate by growing more fleshy tissue. Echinoderms first appeared in
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3822-461: The regular star polygons with intersecting edges that do not generate new vertices, and the other one to the isotoxal concave simple polygons . Polygrams include polygons like the pentagram , but also compound figures like the hexagram . One definition of a star polygon , used in turtle graphics , is a polygon having q ≥ 2 turns ( q is called the turning number or density ), like in spirolaterals . Star polygon names combine
3913-720: The saponins present in their body walls, which have unpleasant flavours. Some starfish such as Astropecten polyacanthus also include powerful toxins such as tetrodotoxin among their chemical armoury, and the slime star can ooze out large quantities of repellent mucus. They also have body armour in the form of hard plates and spines. The crown-of-thorns starfish is particularly unattractive to potential predators, being heavily defended by sharp spines, laced with toxins and sometimes with bright warning colours . Other species protect their vulnerable tube feet and arm tips by lining their ambulacral grooves with spines and heavily plating their extremities. Several species sometimes suffer from
4004-430: The seabed in all the world's oceans , from warm, tropical zones to frigid, polar regions . They are found from the intertidal zone down to abyssal depths, at 6,000 m (20,000 ft) below the surface. Starfish are marine invertebrates . They typically have a central disc and usually five arms, though some species have a larger number of arms. The aboral or upper surface may be smooth, granular or spiny, and
4095-419: The 5th vertex, from the 5th to the 2nd vertex, from the 2nd to the 4th vertex, and from the 4th to the 1st vertex. If q ≥ p /2, then the construction of { p / q } will result in the same polygon as { p /( p − q )}; connecting every third vertex of the pentagon will yield an identical result to that of connecting every second vertex. However, the vertices will be reached in the opposite direction, which makes
4186-592: The Invasive Species Specialist Group's list of the world's 100 worst invasive species . Sea Stars (starfish) are the main predators of kelp-eating sea urchins. Satellite imagery shows that sea urchin populations have exploded due to starfish mass deaths, and that by 2021, sea urchins have destroyed 95% of California's kelp forests. Starfish may be preyed on by conspecifics, sea anemones, other starfish species, tritons , crabs, fish, gulls and sea otters . Their first lines of defence are
4277-416: The area of the polygon is calculated, each of these approaches yields a different result. Star polygons feature prominently in art and culture. Such polygons may or may not be regular , but they are always highly symmetrical . Examples include: Myoepithelial cell Myoepithelial cells (sometimes referred to as myoepithelium ) are cells usually found in glandular epithelium as a thin layer above
4368-407: The basement membrane and glandular epithelium. Each cell consists of a cell body from which 4-8 processes radiate and embrace the secretory unit. Myoepithelial cells have contractile functions. They help in expelling secretions from the lumen of secretory units and facilitate the movement of saliva in salivary ducts. List of distinct cell types in the adult human body This anatomy article
4459-494: The body cavity that reach through the body wall and extend into the surrounding water. These serve a respiratory function. The structures are supported by collagen fibres set at right angles to each other and arranged in a three-dimensional web with the ossicles and papulae in the interstices . This arrangement enables both easy flexion of the arms by the starfish and the rapid onset of stiffness and rigidity required for actions performed under stress. The water vascular system of
4550-621: The body. Starfish produce a large number of secondary metabolites in the form of lipids , including steroidal derivatives of cholesterol , and fatty acid amides of sphingosine . The steroids are mostly saponins , known as asterosaponins, and their sulphated derivatives. They vary between species and are typically formed from up to six sugar molecules (usually glucose and galactose ) connected by up to three glycosidic chains. Long-chain fatty acid amides of sphingosine occur frequently and some of them have known pharmacological activity . Various ceramides are also known from starfish and
4641-808: The case of wound healing, myoepithelial cells reactively proliferate . Presence of myoepithelial cells in a hyperplastic tissue proves the benignity of the gland and, when absent, indicates cancer . Only rare cancers like adenoid cystic carcinomas contains myoepithelial cells as one of the malignant components. It can be found in endoderm or ectoderm . Myoepithelial cells are true epithelial cells positive for keratins , not to be confused with myofibroblasts which are true mesenchymal cells positive for vimentin . These cells are generally positive for alpha smooth muscle actin (αSMA), cytokeratin 5/6 and other high molecular weight cytokeratins, p63 and caldesmon . Myoepithelial cells are stellate in shape and are also known as basket cells. They lie between
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#17327870178114732-548: The deep-sea floor down to at least 6,000 m (20,000 ft). The greatest diversity of species occurs in coastal areas. Most species are generalist predators, eating microalgae , sponges , bivalves , snails and other small animals. The crown-of-thorns starfish consumes coral polyps, while other species are detritivores , feeding on decomposing organic material and faecal matter. A few are suspension feeders, gathering in phytoplankton ; Henricia and Echinaster often occur in association with sponges, benefiting from
4823-542: The density q and amount p of vertices are not coprime. When constructing star polygons from stellation, however, if q > p /2, the lines will instead diverge infinitely, and if q = p /2, the lines will be parallel, with both resulting in no further intersection in Euclidean space. However, it may be possible to construct some such polygons in spherical space, similarly to the monogon and digon ; such polygons do not yet appear to have been studied in detail. When
4914-527: The division of the body may happen inadvertently due to part being detached by a predator, or part may be actively shed by the starfish in an escape response. The loss of parts of the body is achieved by the rapid softening of a special type of connective tissue in response to nervous signals. This type of tissue is called catch connective tissue and is found in most echinoderms. An autotomy-promoting factor has been identified which, when injected into another starfish, causes rapid shedding of arms. The lifespan of
5005-442: The expression patterns of genes that express differently across the body axes suggests that one could think of the body of a starfish as a disembodied head walking about the sea floor on its lips. The known markers for trunk structures are expressed only in internal tissues rather than on the surface. Only the front part of the axis, which specifies head-related structures, is represented on the body surface. The body wall consists of
5096-432: The feeding and growth rates of P. ochraceus reduce greatly when their body temperatures rise above 23 °C (73 °F) and that they die when their temperature rises to 30 °C (86 °F). This species has a unique ability to absorb seawater to keep itself cool when it is exposed to sunlight by a receding tide. It also appears to rely on its arms to absorb heat, so as to protect the central disc and vital organs like
5187-401: The first one, and continuing the process until the original vertex is reached again. Alternatively, for integers p and q , it can be considered as being constructed by connecting every q th point out of p points regularly spaced in a circular placement. For instance, in a regular pentagon, a five-pointed star can be obtained by drawing a line from the 1st to the 3rd vertex, from the 3rd to
5278-574: The genus. Among starfish that are able to regenerate their whole body from a single arm, some can do so even from fragments just 1 cm (0.4 in) long. Single arms that regenerate a whole individual are called comet forms. The division of the starfish, either across its disc or at the base of the arm, is usually accompanied by a weakness in the structure that provides a fracture zone. The larvae of several species of starfish can reproduce asexually before they reach maturity. They do this by autotomising some parts of their bodies or by budding . When such
5369-569: The interior of the gonads themselves. Those starfish that brood their eggs by "sitting" on them usually assume a humped posture with their discs raised off the substrate. Pteraster militaris broods a few of its young and disperses the remaining eggs, that are too numerous to fit into its pouch. In these brooding species, the eggs are relatively large, and supplied with yolk , and they generally develop directly into miniature starfish without an intervening larval stage. The developing young are called lecithotrophic because they obtain their nutrition from
5460-722: The intersecting line segments are removed from a regular star n -gon, the resulting figure is no longer regular, but can be seen as an isotoxal concave simple 2 n -gon, alternating vertices at two different radii. Branko Grünbaum , in Tilings and patterns , represents such a star that matches the outline of a regular polygram { n / d } as | n / d |, or more generally with { n 𝛼 }, which denotes an isotoxal concave or convex simple 2 n -gon with outer internal angle 𝛼. These polygons are often seen in tiling patterns. The parametric angle 𝛼 (in degrees or radians) can be chosen to match internal angles of neighboring polygons in
5551-403: The lack of an osmoregulation system probably explains why starfish are not found in fresh water or even in many estuarine environments. Although starfish do not have many well-defined sense organs, they are sensitive to touch, light, temperature, orientation and the status of the water around them. The tube feet, spines and pedicellariae are sensitive to touch. The tube feet, especially those at
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#17327870178115642-428: The late Paleozoic , the crinoids and blastoids were the predominant echinoderms, and some limestones from this period are made almost entirely from fragments from these groups. In the two major extinction events that occurred during the late Devonian and late Permian , the blastoids were wiped out and only a few species of crinoids survived. Many starfish species also became extinct in these events, but afterwards
5733-649: The latter remaining a broadcast spawner. The scientific name Asteroidea was given to starfish by the French zoologist de Blainville in 1830. It is derived from the Greek aster , ἀστήρ (a star) and the Greek eidos , εἶδος (form, likeness, appearance). The class Asteroidea belongs to the phylum Echinodermata . As well as the starfish, the echinoderms include sea urchins , sand dollars , brittle and basket stars , sea cucumbers and crinoids . The larvae of echinoderms have bilateral symmetry, but during metamorphosis this
5824-402: The leading arms, while others trail behind. Most starfish cannot move quickly, a typical speed being that of the leather star ( Dermasterias imbricata ), which can manage just 15 cm (6 in) in a minute. Some burrowing species from the genera Astropecten and Luidia have points rather than suckers on their long tube feet and are capable of much more rapid motion, "gliding" across
5915-458: The mouth. The next stage in development is a brachiolaria larva and involves the growth of three short, additional arms. These are at the anterior end, surround a sucker and have adhesive cells at their tips. Both bipinnaria and brachiolaria larvae are bilaterally symmetrical. When fully developed, the brachiolaria settles on the seabed and attaches itself with a short stalk formed from the ventral arms and sucker. Metamorphosis now takes place with
6006-463: The mouth. Another invagination of the surface will fuse with the tip of the archenteron as the mouth while the interior section will become the gut. At the same time, a band of cilia develops on the exterior. This enlarges and extends around the surface and eventually onto two developing arm-like outgrowths. At this stage the larva is known as a bipinnaria . The cilia are used for locomotion and feeding, their rhythmic beat wafting phytoplankton towards
6097-469: The ocean floor. The sand star ( Luidia foliolata ) can travel at a speed of 2.8 m (9 ft 2 in) per minute. When a starfish finds itself upside down, two adjacent arms are bent backwards to provide support, the opposite arm is used to stamp the ground while the two remaining arms are raised on either side; finally the stamping arm is released as the starfish turns itself over and recovers its normal stance. Apart from their function in locomotion,
6188-403: The other arms to initiate movement towards the prey. The mechanism for this is not fully understood. The body cavity contains the circulatory or haemal system. The vessels form three rings: one around the mouth (the hyponeural haemal ring), another around the digestive system (the gastric ring) and the third near the aboral surface (the genital ring). The heart beats about six times a minute and
6279-666: The prey. The stomach and the partially digested prey are later retracted into the disc. Here the food is passed on to the pyloric stomach, which always remains inside the disc. The retraction and contraction of the cardiac stomach is activated by a neuropeptide known as NGFFYamide. Because of this ability to digest food outside the body, starfish can hunt prey much larger than their mouths. Their diets include clams and oysters , arthropods , small fish and gastropod molluscs . Some starfish are not pure carnivores , supplementing their diets with algae or organic detritus. Some of these species are grazers , but others trap food particles from
6370-540: The pyloric caeca. These are elongated, branched hollow tubes that are lined by a series of glands, which secrete digestive enzymes and absorb nutrients from the food. A short intestine and rectum run from the pyloric stomach to open at a small anus at the apex of the aboral surface of the disc. Primitive starfish, such as Astropecten and Luidia , swallow their prey whole, and start to digest it in their cardiac stomachs. Shell valves and other inedible materials are ejected through their mouths. The semi-digested fluid
6461-418: The release of which trigger responses in conspecific starfish but often produce escape responses in potential prey. Research into the efficacy of these compounds for possible pharmacological or industrial use occurs worldwide. Most species of starfish are gonochorous , there being separate male and female individuals. These are usually not distinguishable externally as the gonads cannot be seen, but their sex
6552-428: The resemblance of these shapes to the diffraction spikes of real stars. A regular star polygon is a self-intersecting, equilateral, and equiangular polygon . A regular star polygon is denoted by its Schläfli symbol { p / q }, where p (the number of vertices) and q (the density ) are relatively prime (they share no factors) and where q ≥ 2. The density of a polygon can also be called its turning number :
6643-451: The sea water. Most starfish embryos hatch at the blastula stage. The original ball of cells develops a lateral pouch, the archenteron . The entrance to this is known as the blastopore and it will later develop into the anus—together with chordates , echinoderms are deuterostomes , meaning the second ( deutero ) invagination becomes the mouth ( stome ); members of all other phyla are protostomes , and their first invagination becomes
6734-671: The soft substrate to extract prey (usually clams ). Grasping the shellfish, the starfish slowly pries open the prey's shell by wearing out its adductor muscle, and then inserts its everted stomach into the crack to digest the soft tissues. The gap between the valves need only be a fraction of a millimetre wide for the stomach to gain entry. Cannibalism has been observed in juvenile sea stars as early as four days after metamorphosis. Starfish are keystone species in their respective marine communities . Their relatively large sizes, diverse diets and ability to adapt to different environments makes them ecologically important. The term "keystone species"
6825-416: The species. While a starfish lacks a centralized brain , it has a complex nervous system with a nerve ring around the mouth and a radial nerve running along the ambulacral region of each arm parallel to the radial canal. The peripheral nerve system consists of two nerve nets: a sensory system in the epidermis and a motor system in the lining of the coelomic cavity. Neurons passing through the dermis connect
6916-467: The starfish is a hydraulic system made up of a network of fluid-filled canals and is concerned with locomotion, adhesion, food manipulation and gas exchange . Water enters the system through the madreporite , a porous, often conspicuous, sieve-like ossicle on the aboral surface. It is linked through a stone canal, often lined with calcareous material, to a ring canal around the mouth opening. A set of radial canals leads off this; one radial canal runs along
7007-409: The stomach. Starfish and other echinoderms are sensitive to marine pollution . The common starfish is considered to be a bioindicator for marine ecosystems. A 2009 study found that P. ochraceus is unlikely to be affected by ocean acidification as severely as other marine animals with calcareous skeletons. In other groups, structures made of calcium carbonate are vulnerable to dissolution when
7098-449: The substrate. The tube feet latch on to surfaces and move in a wave, with one arm section attaching to the surface as another releases. Some starfish turn up the tips of their arms while moving which gives maximum exposure of the sensory tube feet and the eyespot to external stimuli. Having descended from bilateral organisms, starfish may move in a bilateral fashion, particularly when hunting or in danger. When crawling, certain arms act as
7189-423: The sum of the turn angles of all the vertices, divided by 360°. The symmetry group of { p / q } is the dihedral group D p , of order 2 p , independent of q . Regular star polygons were first studied systematically by Thomas Bradwardine , and later Johannes Kepler . Regular star polygons can be created by connecting one vertex of a regular p -sided simple polygon to another vertex, non-adjacent to
7280-549: The surviving few species diversified rapidly within about sixty million years during the Early Jurassic and the beginning of the Middle Jurassic . A 2012 study found that speciation in starfish can occur rapidly. During the last 6,000 years, divergence in the larval development of Cryptasterina hystera and Cryptasterina pentagona has taken place, the former adopting internal fertilization and brooding and
7371-407: The tips of the papulae, where a portion of body wall is nipped off and ejected into the surrounding water. Some waste may also be excreted by the pyloric glands and voided with the faeces . Starfish do not appear to have any mechanisms for osmoregulation , and keep their body fluids at the same salt concentration as the surrounding water. Although some species can tolerate relatively low salinity ,
7462-568: The tips of the rays, are also sensitive to chemicals, enabling the starfish to detect odour sources such as food. There are eyespots at the ends of the arms, each one made of 80–200 simple ocelli . These are composed of pigmented epithelial cells that respond to light and are covered by a thick, transparent cuticle that both protects the ocelli and acts to focus light. Many starfish also possess individual photoreceptor cells in other parts of their bodies and respond to light even when their eyespots are covered. Whether they advance or retreat depends on
7553-471: The tropics, a plentiful supply of phytoplankton is continuously available for starfish larvae to feed on. Spawning takes place at any time of year, each species having its own characteristic breeding season. In temperate regions, the spring and summer brings an increase in food supplies. The first individual of a species to spawn may release a pheromone that serves to attract other starfish to aggregate and to release their gametes synchronously. In other species,
7644-414: The tube feet act as accessory gills. The water vascular system serves to transport oxygen from, and carbon dioxide to, the tube feet and also nutrients from the gut to the muscles involved in locomotion. Fluid movement is bidirectional and initiated by cilia . Gas exchange also takes place through other gills known as papulae, which are thin-walled bulges on the aboral surface of the disc and arms. Oxygen
7735-410: The two. The ring nerves and radial nerves have sensory and motor components and coordinate the starfish's balance and directional systems. The sensory component receives input from the sensory organs while the motor nerves control the tube feet and musculature. The starfish does not have the capacity to plan its actions. If one arm detects an attractive odour, it becomes dominant and temporarily over-rides
7826-433: The water (free spawning) and the resulting embryos and larvae live as part of the plankton . In others, the eggs may be stuck to the undersides of rocks. In certain species of starfish, the females brood their eggs – either by simply enveloping them or by holding them in specialised structures. Brooding may be done in pockets on the starfish's aboral surface, inside the pyloric stomach ( Leptasterias tenera ) or even in
7917-462: The water current they produce. Various species have been shown to be able to absorb organic nutrients from the surrounding water, and this may form a significant portion of their diet. The processes of feeding and capture may be aided by special parts; Pisaster brevispinus , the short-spined pisaster from the West Coast of America, can use a set of specialized tube feet to dig itself deep into
8008-453: The water in sticky mucus strands that are swept towards the mouth along ciliated grooves. The main nitrogenous waste product is ammonia . Starfish have no distinct excretory organs; waste ammonia is removed by diffusion through the tube feet and papulae. The body fluid contains phagocytic cells called coelomocytes, which are also found within the hemal and water vascular systems. These cells engulf waste material, and eventually migrate to
8099-422: The whole canal system is lined with cilia . When longitudinal muscles in the ampullae contract, valves in the lateral canals close and water is forced into the tube feet. These extend to contact the substrate . Although the tube feet resemble suction cups in appearance, the gripping action is a function of adhesive chemicals rather than suction. Other chemicals and relaxation of the ampullae allow for release from
8190-407: The yolk as opposed to "planktotrophic" larvae that feed in the water column . In Parvulastra parvivipara , an intragonadal brooder, the young starfish obtain nutrients by eating other eggs and embryos in the brood pouch. Brooding is especially common in polar and deep-sea species that live in environments unfavourable for larval development and in smaller species that produce just a few eggs. In
8281-515: Was in fact first used by Robert Paine in 1966 to describe a starfish, Pisaster ochraceus . When studying the low intertidal coasts of Washington state , Paine found that predation by P. ochraceus was a major factor in the diversity of species. Experimental removals of this top predator from a stretch of shoreline resulted in lower species diversity and the eventual domination of Mytilus mussels, which were able to outcompete other organisms for space and resources. Similar results were found in
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