116-519: See taxonomy An echinoderm ( / ɪ ˈ k aɪ n ə ˌ d ɜːr m , ˈ ɛ k ə -/ ) 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
232-571: A clade , meaning that they arose from a single common ancestor . Over 1.5 million living animal species have been described , of which around 1.05 million are insects , over 85,000 are molluscs , and around 65,000 are vertebrates . It has been estimated there are as many as 7.77 million animal species on Earth. Animal body lengths range from 8.5 μm (0.00033 in) to 33.6 m (110 ft). They have complex ecologies and interactions with each other and their environments, forming intricate food webs . The scientific study of animals
348-548: 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
464-399: A gastrula with a digestive chamber and two separate germ layers , an external ectoderm and an internal endoderm . In most cases, a third germ layer, the mesoderm , also develops between them. These germ layers then differentiate to form tissues and organs. Repeated instances of mating with a close relative during sexual reproduction generally leads to inbreeding depression within
580-425: 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
696-561: A clade Xenambulacraria for the Xenacoelamorpha + Ambulacraria; this is either within Deuterostomia, as sister to Chordata, or the Deuterostomia are recovered as paraphyletic, and Xenambulacraria is sister to the proposed clade Centroneuralia , consisting of Chordata + Protostomia. Eumetazoa , a clade which contains Ctenophora and ParaHoxozoa , has been proposed as a sister group to Porifera . A competing hypothesis
812-432: 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,
928-401: 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
1044-434: A few different reasons. When a predator is near, certain species of sand dollar larvae will split themselves in half in a process they use to asexually clone themselves when sensing danger. The cloning process can take up to 24 hours and creates larvae that are 2/3 smaller than their original size which can help conceal them from the predator. The larvae of these sand dollars clone themselves when they sense dissolved mucus from
1160-407: A fivefold symmetric pattern. The test of certain species of sand dollar have slits called lunules that can help the animal stay embedded in the sand to stop it from being swept away by an ocean wave. In living individuals, the test is covered by a skin of velvet -textured spines which are covered with very small hairs ( cilia ). Coordinated movements of the spines enable sand dollars to move across
1276-472: 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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#17327944594081392-618: 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
1508-475: 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
1624-1109: A population due to the increased prevalence of harmful recessive traits. Animals have evolved numerous mechanisms for avoiding close inbreeding . Some animals are capable of asexual reproduction , which often results in a genetic clone of the parent. This may take place through fragmentation ; budding , such as in Hydra and other cnidarians ; or parthenogenesis , where fertile eggs are produced without mating , such as in aphids . Animals are categorised into ecological groups depending on their trophic levels and how they consume organic material . Such groupings include carnivores (further divided into subcategories such as piscivores , insectivores , ovivores , etc.), herbivores (subcategorized into folivores , graminivores , frugivores , granivores , nectarivores , algivores , etc.), omnivores , fungivores , scavengers / detritivores , and parasites . Interactions between animals of each biome form complex food webs within that ecosystem . In carnivorous or omnivorous species, predation
1740-428: 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
1856-451: A predatory fish. The larvae exposed to this mucus from the predatory fish respond to the threat by cloning themselves. This process doubles their population and halves their size which allows them to better escape detection by the predatory fish but may make them more vulnerable to attacks from smaller predators like crustaceans. Sand dollars will also clone themselves during normal asexual reproduction. Larvae will undergo this process when food
1972-449: 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
2088-540: 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
2204-582: A single opening, which serves as both mouth and anus. Animals in both phyla have distinct tissues, but these are not organised into discrete organs . They are diploblastic , having only two main germ layers, ectoderm and endoderm. Sand dollar See text. Sand dollars (also known as sea cookies or snapper biscuits in New Zealand and Brazil , or pansy shells in South Africa ) are species of flat, burrowing sea urchins belonging to
2320-536: 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
2436-458: 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
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#17327944594082552-540: 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
2668-464: Is a consumer–resource interaction where the predator feeds on another organism, its prey , who often evolves anti-predator adaptations to avoid being fed upon. Selective pressures imposed on one another lead to an evolutionary arms race between predator and prey, resulting in various antagonistic/ competitive coevolutions . Almost all multicellular predators are animals. Some consumers use multiple methods; for example, in parasitoid wasps ,
2784-446: 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
2900-433: Is a stage in embryonic development that is unique to animals, allowing cells to be differentiated into specialised tissues and organs. All animals are composed of cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins . During development, the animal extracellular matrix forms a relatively flexible framework upon which cells can move about and be reorganised, making
3016-412: 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
3132-463: Is disputed, as they might be water-escape or other structures. Animals are monophyletic , meaning they are derived from a common ancestor. Animals are the sister group to the choanoflagellates , with which they form the Choanozoa . The dates on the phylogenetic tree indicate approximately how many millions of years ago ( mya ) the lineages split. Ros-Rocher and colleagues (2021) trace
3248-495: 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
3364-450: Is known as zoology , and the study of animal behaviour is known as ethology . The animal kingdom is divided into five infrakingdoms / superphyla , namely Porifera , Ctenophora , Placozoa , Cnidaria and Bilateria . Most living animal species belong to the infrakingdom Bilateria, a highly proliferative clade whose members have a bilaterally symmetric body plan , and the vast majority of bilaterians belong to two large superphyla :
3480-426: 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
3596-402: 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
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3712-808: Is often used to refer only to nonhuman animals. The term metazoa is derived from Ancient Greek μετα ( meta ) 'after' (in biology, the prefix meta- stands for 'later') and ζῷᾰ ( zōia ) 'animals', plural of ζῷον zōion 'animal'. Animals have several characteristics that set them apart from other living things. Animals are eukaryotic and multicellular . Unlike plants and algae , which produce their own nutrients , animals are heterotrophic , feeding on organic material and digesting it internally. With very few exceptions, animals respire aerobically . All animals are motile (able to spontaneously move their bodies) during at least part of their life cycle , but some animals, such as sponges , corals , mussels , and barnacles , later become sessile . The blastula
3828-422: Is only produced by sponges and pelagophyte algae. Its likely origin is from sponges based on molecular clock estimates for the origin of 24-ipc production in both groups. Analyses of pelagophyte algae consistently recover a Phanerozoic origin, while analyses of sponges recover a Neoproterozoic origin, consistent with the appearance of 24-ipc in the fossil record. The first body fossils of animals appear in
3944-511: Is plentiful or temperature conditions are optimal. Cloning may also occur to make use of the tissues that are normally lost during metamorphosis. The flattened test of the sand dollar allows it to burrow into the sand and remain hidden from sight from potential predators. Predators of the sand dollar are the fish species cod, flounder, sheepshead and haddock. These fish will prey on sand dollars even through their tough exterior. Sand dollars have spines on their bodies that help them to move around
4060-440: 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
4176-481: 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
4292-547: Is the Benthozoa clade, which would consist of Porifera and ParaHoxozoa as a sister group of Ctenophora . Several animal phyla lack bilateral symmetry. These are the Porifera (sea sponges), Placozoa , Cnidaria (which includes jellyfish , sea anemones , and corals), and Ctenophora (comb jellies). Sponges are physically very distinct from other animals, and were long thought to have diverged first, representing
4408-590: Is thicker in height than most. In Spanish-speaking areas of the Americas, the sand dollar is most often known as galleta de mar (sea cookie ); the translated term is often encountered in English. In the folklore of Georgia in the United States, sand dollars were believed to represent coins lost by mermaids . Sand dollars diverged from the other irregular echinoids, namely the cassiduloids , during
4524-622: The Cambrian explosion , which began around 539 million years ago (Mya), and most classes during the Ordovician radiation 485.4 Mya. 6,331 groups of genes common to all living animals have been identified; these may have arisen from a single common ancestor that lived about 650 Mya during the Cryogenian period. Historically, Aristotle divided animals into those with blood and those without . Carl Linnaeus created
4640-619: The Ediacaran , represented by forms such as Charnia and Spriggina . It had long been doubted whether these fossils truly represented animals, but the discovery of the animal lipid cholesterol in fossils of Dickinsonia establishes their nature. Animals are thought to have originated under low-oxygen conditions, suggesting that they were capable of living entirely by anaerobic respiration , but as they became specialized for aerobic metabolism they became fully dependent on oxygen in their environments. Many animal phyla first appear in
4756-485: The Wnt and TGF-beta signalling pathways which may have enabled animals to become multicellular by providing a pattern for the body's system of axes (in three dimensions), and another 7 are for transcription factors including homeodomain proteins involved in the control of development . Giribet and Edgecombe (2020) provide what they consider to be a consensus internal phylogeny of the animals, embodying uncertainty about
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4872-552: 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
4988-406: The endoskeleton through which podia for gas exchange project from the body. The mouth of the sand dollar is located on the bottom of its body at the center of the petal-like pattern. Unlike other urchins, the bodies of sand dollars also display secondary front-to-back bilateral symmetry with no morphological distinguishing features between males and females. The anus of sand dollars is located at
5104-665: The evolutionary relationships between taxa . Humans make use of many other animal species for food (including meat , eggs , and dairy products ), for materials (such as leather , fur , and wool ), as pets and as working animals for transportation , and services . Dogs , the first domesticated animal, have been used in hunting , in security and in warfare , as have horses , pigeons and birds of prey ; while other terrestrial and aquatic animals are hunted for sports, trophies or profits. Non-human animals are also an important cultural element of human evolution , having appeared in cave arts and totems since
5220-473: The fossil record during the Cambrian explosion , starting about 539 million years ago, in beds such as the Burgess shale . Extant phyla in these rocks include molluscs , brachiopods , onychophorans , tardigrades , arthropods , echinoderms and hemichordates , along with numerous now-extinct forms such as the predatory Anomalocaris . The apparent suddenness of the event may however be an artifact of
5336-497: The lobe-finned fish Tiktaalik started to move on to land in the late Devonian , about 375 million years ago. Animals occupy virtually all of earth's habitats and microhabitats, with faunas adapted to salt water, hydrothermal vents, fresh water, hot springs, swamps, forests, pastures, deserts, air, and the interiors of other organisms. Animals are however not particularly heat tolerant ; very few of them can survive at constant temperatures above 50 °C (122 °F) or in
5452-757: 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
5568-412: The order Clypeasteroida . Some species within the order, not quite as flat, are known as sea biscuits . Sand dollars can also be called "sand cakes" or "cake urchins". The term "sand dollar" derives from the appearance of the tests (skeletons) of dead individuals after being washed ashore. The test lacks its velvet-like skin of spines and has often been bleached white by sunlight . To beachcombers of
5684-500: The protostomes , which includes organisms such as arthropods , molluscs , flatworms , annelids and nematodes ; and the deuterostomes , which include echinoderms , hemichordates and chordates , the latter of which contains the vertebrates . The much smaller basal phylum Xenacoelomorpha have an uncertain position within Bilateria. Animals first appear in the fossil record in the late Cryogenian period and diversified in
5800-425: 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
5916-602: 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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#17327944594086032-715: The Cambrian explosion, possibly as early as 1 billion years ago. Early fossils that might represent animals appear for example in the 665-million-year-old rocks of the Trezona Formation of South Australia . These fossils are interpreted as most probably being early sponges . Trace fossils such as tracks and burrows found in the Tonian period (from 1 gya) may indicate the presence of triploblastic worm-like animals, roughly as large (about 5 mm wide) and complex as earthworms. However, similar tracks are produced by
6148-717: The Cnidaria) never grow larger than 20 μm , and one of the smallest species ( Myxobolus shekel ) is no more than 8.5 μm when fully grown. The following table lists estimated numbers of described extant species for the major animal phyla, along with their principal habitats (terrestrial, fresh water, and marine), and free-living or parasitic ways of life. Species estimates shown here are based on numbers described scientifically; much larger estimates have been calculated based on various means of prediction, and these can vary wildly. For instance, around 25,000–27,000 species of nematodes have been described, while published estimates of
6264-727: 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:
6380-421: 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
6496-593: 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,
6612-536: 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
6728-510: The back rather than at the top as in most urchins, with many more bilateral features appearing in some species. These result from the adaptation of sand dollars, in the course of their evolution , from creatures that originally lived their lives on top of the seabed ( epibenthos ) to creatures that burrow beneath it ( endobenthos ). According to World Register of Marine Species : Sand dollars can be found in temperate and tropical zones along all continents. Sand dollars live in waters below
6844-484: 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
6960-420: 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
7076-444: 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
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#17327944594087192-487: 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
7308-413: 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
7424-463: The dark sea floor consume organic matter produced through chemosynthesis (via oxidizing inorganic compounds such as hydrogen sulfide ) by archaea and bacteria . Animals evolved in the sea. Lineages of arthropods colonised land around the same time as land plants , probably between 510 and 471 million years ago during the Late Cambrian or Early Ordovician . Vertebrates such as
7540-454: The earliest times, and are frequently featured in mythology , religion , arts , literature , heraldry , politics , and sports . The word animal comes from the Latin noun animal of the same meaning, which is itself derived from Latin animalis 'having breath or soul'. The biological definition includes all members of the kingdom Animalia. In colloquial usage, the term animal
7656-535: The early Jurassic , with the first true sand dollar genus, Togocyamus , arising during the Paleocene . Soon after Togocyamus , more modern-looking groups emerged during the Eocene . Sand dollars are small in size, averaging from 80 to 100 mm (3 to 4 inches). As with all members of the order Clypeasteroida, they possess a rigid skeleton called a test . The test consists of calcium carbonate plates arranged in
7772-451: 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
7888-570: The first hierarchical biological classification for animals in 1758 with his Systema Naturae , which Jean-Baptiste Lamarck expanded into 14 phyla by 1809. In 1874, Ernst Haeckel divided the animal kingdom into the multicellular Metazoa (now synonymous with Animalia) and the Protozoa , single-celled organisms no longer considered animals. In modern times, the biological classification of animals relies on advanced techniques, such as molecular phylogenetics , which are effective at demonstrating
8004-446: 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
8120-448: The formation of complex structures possible. This may be calcified, forming structures such as shells , bones , and spicules . In contrast, the cells of other multicellular organisms (primarily algae, plants, and fungi ) are held in place by cell walls, and so develop by progressive growth. Animal cells uniquely possess the cell junctions called tight junctions , gap junctions , and desmosomes . With few exceptions—in particular,
8236-520: The fossil record, rather than showing that all these animals appeared simultaneously. That view is supported by the discovery of Auroralumina attenboroughii , the earliest known Ediacaran crown-group cnidarian (557–562 mya, some 20 million years before the Cambrian explosion) from Charnwood Forest , England. It is thought to be one of the earliest predators , catching small prey with its nematocysts as modern cnidarians do. Some palaeontologists have suggested that animals appeared much earlier than
8352-440: 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
8468-492: 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
8584-737: The giant single-celled protist Gromia sphaerica , so the Tonian trace fossils may not indicate early animal evolution. Around the same time, the layered mats of microorganisms called stromatolites decreased in diversity, perhaps due to grazing by newly evolved animals. Objects such as sediment-filled tubes that resemble trace fossils of the burrows of wormlike animals have been found in 1.2 gya rocks in North America, in 1.5 gya rocks in Australia and North America, and in 1.7 gya rocks in Australia. Their interpretation as having an animal origin
8700-426: The global carbon cycle . Animal Animals are multicellular , eukaryotic organisms in the biological kingdom Animalia ( / ˌ æ n ɪ ˈ m eɪ l i ə / ). With few exceptions, animals consume organic material , breathe oxygen , have myocytes and are able to move , can reproduce sexually , and grow from a hollow sphere of cells, the blastula , during embryonic development . Animals form
8816-534: 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 the erosion produced by the sea urchin. Echinoderms sequester about 0.1 gigatonnes of carbon dioxide per year as calcium carbonate , making them important contributors in
8932-415: 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
9048-432: 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 the ecological roles of adults are
9164-441: The larvae feed on the hosts' living tissues, killing them in the process, but the adults primarily consume nectar from flowers. Other animals may have very specific feeding behaviours , such as hawksbill sea turtles which mainly eat sponges . Most animals rely on biomass and bioenergy produced by plants and phytoplanktons (collectively called producers ) through photosynthesis . Herbivores, as primary consumers , eat
9280-516: 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
9396-455: 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
9512-771: 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
9628-477: 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
9744-534: The mean low tide line, on or just beneath the surface of sandy and muddy areas. The common sand dollar, Echinarachnius parma , can be found in the Northern Hemisphere from the intertidal zone to the depths of the ocean, while the keyhole sand dollars (three species of the genus Mellita ) can be found on many a wide range of coasts in and around the Caribbean Sea . The spines on
9860-753: The most extreme cold deserts of continental Antarctica . The blue whale ( Balaenoptera musculus ) is the largest animal that has ever lived, weighing up to 190 tonnes and measuring up to 33.6 metres (110 ft) long. The largest extant terrestrial animal is the African bush elephant ( Loxodonta africana ), weighing up to 12.25 tonnes and measuring up to 10.67 metres (35.0 ft) long. The largest terrestrial animals that ever lived were titanosaur sauropod dinosaurs such as Argentinosaurus , which may have weighed as much as 73 tonnes, and Supersaurus which may have reached 39 meters. Several animals are microscopic; some Myxozoa ( obligate parasites within
9976-533: 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
10092-534: 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
10208-478: The ocean floor, in part to their preference for soft bottom areas, which are convenient for their reproduction . The sexes are separate and, as with most echinoids, gametes are released into the water column and go through external fertilization . The nektonic larvae metamorphose through several stages before the skeleton or test begins to form, at which point they become benthic . In 2008, biologists discovered that sand dollar larvae will clone themselves for
10324-464: 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
10440-620: The oldest animal phylum and forming a sister clade to all other animals. Despite their morphological dissimilarity with all other animals, genetic evidence suggests sponges may be more closely related to other animals than the comb jellies are. Sponges lack the complex organization found in most other animal phyla; their cells are differentiated, but in most cases not organised into distinct tissues, unlike all other animals. They typically feed by drawing in water through pores, filtering out small particles of food. The comb jellies and Cnidaria are radially symmetric and have digestive chambers with
10556-529: 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
10672-525: The origins of animals to unicellular ancestors, providing the external phylogeny shown in the cladogram. Uncertainty of relationships is indicated with dashed lines. Holomycota (inc. fungi) [REDACTED] Ichthyosporea [REDACTED] Pluriformea [REDACTED] Filasterea [REDACTED] [REDACTED] [REDACTED] The most basal animals, the Porifera , Ctenophora , Cnidaria , and Placozoa , have body plans that lack bilateral symmetry . Their relationships are still disputed;
10788-421: 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
10904-418: The past, this suggested a large, silver coin, such as the old Spanish dollar , which had a diameter of 38–40 mm. Other names for the sand dollar include sand cakes, pansy shells, snapper biscuits, cake urchins, and sea cookies. In South Africa, they are known as pansy shells from their suggestion of a five-petaled garden flower . The Caribbean sand dollar or inflated sea biscuit, Clypeaster rosaceus ,
11020-519: The plant material directly to digest and absorb the nutrients, while carnivores and other animals on higher trophic levels indirectly acquire the nutrients by eating the herbivores or other animals that have eaten the herbivores. Animals oxidize carbohydrates , lipids , proteins and other biomolecules, which allows the animal to grow and to sustain basal metabolism and fuel other biological processes such as locomotion . Some benthic animals living close to hydrothermal vents and cold seeps on
11136-422: 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
11252-455: 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
11368-457: 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
11484-447: 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
11600-465: 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
11716-440: 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
11832-606: The seabed. The velvety spines of live sand dollars appear in a variety of colors—green, blue, violet, or purple—depending on the species. Individuals which are very recently dead or dying (moribund) are sometimes found on beaches with much of the external morphology still intact. Dead individuals are commonly found with their empty test devoid of all surface material and bleached white by sunlight. The bodies of adult sand dollars, like those of other echinoids , display radial symmetry . The petal-like pattern in sand dollars consists of five paired rows of pores. The pores are perforations in
11948-753: The sister group to all other animals could be the Porifera or the Ctenophora, both of which lack hox genes , which are important for body plan development . Hox genes are found in the Placozoa, Cnidaria, and Bilateria. 6,331 groups of genes common to all living animals have been identified; these may have arisen from a single common ancestor that lived 650 million years ago in the Precambrian . 25 of these are novel core gene groups, found only in animals; of those, 8 are for essential components of
12064-414: 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,
12180-407: The smaller, motile gametes are spermatozoa and the larger, non-motile gametes are ova . These fuse to form zygotes , which develop via mitosis into a hollow sphere, called a blastula. In sponges, blastula larvae swim to a new location, attach to the seabed, and develop into a new sponge. In most other groups, the blastula undergoes more complicated rearrangement. It first invaginates to form
12296-648: 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;
12412-400: The somewhat flattened topside and underside of the animal allow it to burrow or creep through the sediment when looking for shelter or food. Fine, hair-like cilia cover these tiny spines. Sand dollars usually eat algae and organic matter found along the ocean floor, though some species will tip on their side to catch organic matter floating in ocean currents. Sand dollars frequently gather on
12528-474: The sponges and placozoans —animal bodies are differentiated into tissues . These include muscles , which enable locomotion, and nerve tissues , which transmit signals and coordinate the body. Typically, there is also an internal digestive chamber with either one opening (in Ctenophora, Cnidaria, and flatworms) or two openings (in most bilaterians). Nearly all animals make use of some form of sexual reproduction. They produce haploid gametes by meiosis ;
12644-411: The structure at the base of the tree (dashed lines). Porifera [REDACTED] Ctenophora [REDACTED] Placozoa [REDACTED] Cnidaria [REDACTED] Xenacoelomorpha [REDACTED] Ambulacraria [REDACTED] Chordata [REDACTED] Ecdysozoa [REDACTED] Spiralia [REDACTED] An alternative phylogeny, from Kapli and colleagues (2021), proposes
12760-522: The subsequent Ediacaran period in what is known as the Avalon explosion . Earlier evidence of animals is still controversial; the sponge -like organism Otavia has been dated back to the Tonian period at the start of the Neoproterozoic , but its identity as an animal is heavily contested. Nearly all modern animal phyla became clearly established in the fossil record as marine species during
12876-467: The total number of nematode species include 10,000–20,000; 500,000; 10 million; and 100 million. Using patterns within the taxonomic hierarchy, the total number of animal species—including those not yet described—was calculated to be about 7.77 million in 2011. 3,000–6,500 4,000–25,000 Evidence of animals is found as long ago as the Cryogenian period. 24-Isopropylcholestane (24-ipc) has been found in rocks from roughly 650 million years ago; it
12992-409: 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
13108-429: 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
13224-401: 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
13340-447: 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
13456-536: 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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