110-494: Hemichordata ( / ˌ h ɛ m ɪ k ɔːr ˈ d eɪ t ə / HEM -ih-kor- DAY -tə ) is a phylum which consists of triploblastic , eucoelomate , and bilaterally symmetrical marine deuterostome animals , generally considered the sister group of the echinoderms . They appear in the Lower or Middle Cambrian and include two main classes: Enteropneusta (acorn worms), and Pterobranchia . A third class, Planctosphaeroidea,
220-554: A gastrula or even a blastula stage. New larvae can develop from the preoral hood (a mound like structure above the mouth), the side body wall, the postero-lateral arms, or their rear ends. Cloning is costly to the larva both in resources and in development time. Larvae undergo this process when food is plentiful or temperature conditions are optimal. Cloning may occur to make use of the tissues that are normally lost during metamorphosis. The larvae of some sand dollars clone themselves when they detect dissolved fish mucus, indicating
330-427: A yolk-feeding larva. The provision of a yolk-sac means that smaller numbers of eggs are produced, the larvae have a shorter development period and a smaller dispersal potential, but a greater chance of survival. Echinoderms are globally distributed in almost all depths, latitudes and environments in the ocean. Adults are mainly benthic , living on the seabed, whereas larvae are often pelagic , living as plankton in
440-556: A certain degree of morphological or developmental similarity (the phenetic definition), or a group of organisms with a certain degree of evolutionary relatedness (the phylogenetic definition). Attempting to define a level of the Linnean hierarchy without referring to (evolutionary) relatedness is unsatisfactory, but a phenetic definition is useful when addressing questions of a morphological nature—such as how successful different body plans were. The most important objective measure in
550-407: A character unique to a sub-set of the crown group. Furthermore, organisms in the stem group of a phylum can possess the "body plan" of the phylum without all the characteristics necessary to fall within it. This weakens the idea that each of the phyla represents a distinct body plan. A classification using this definition may be strongly affected by the chance survival of rare groups, which can make
660-433: A co-ordinated way, propelled by the other four arms. During locomotion, the propelling arms can made either snake-like or rowing movements. Starfish move using their tube feet, keeping their arms almost still, including in genera like Pycnopodia where the arms are flexible. The oral surface is covered with thousands of tube feet which move out of time with each other, but not in a metachronal rhythm ; in some way, however,
770-403: A defensive mechanism when attacked. Echinoderms possess a unique water vascular system, a network of fluid-filled canals modified from the coelom (body cavity) that function in gas exchange, feeding, sensory reception and locomotion. This system varies between different classes of echinoderm but typically opens to the exterior through a sieve-like madreporite on the aboral (upper) surface of
880-402: A group ("a self-contained unity"): "perhaps such a real and completely self-contained unity is the aggregate of all species which have gradually evolved from one and the same common original form, as, for example, all vertebrates. We name this aggregate [a] Stamm [i.e., stock] ( Phylon )." In plant taxonomy , August W. Eichler (1883) classified plants into five groups named divisions,
990-602: A group containing Viridiplantae and the algal Rhodophyta and Glaucophyta divisions. The definition and classification of plants at the division level also varies from source to source, and has changed progressively in recent years. Thus some sources place horsetails in division Arthrophyta and ferns in division Monilophyta, while others place them both in Monilophyta, as shown below. The division Pinophyta may be used for all gymnosperms (i.e. including cycads, ginkgos and gnetophytes), or for conifers alone as below. Since
1100-474: A large stomach and a rectum with the anus at the apex of the test. Sea cucumbers are mostly detritivores , sorting through the sediment with modified tube feet around their mouth, the buccal tentacles. Sand and mud accompanies their food through their simple gut, which has a long coiled intestine and a large cloaca . Crinoids are suspension feeders , passively catching plankton which drift into their outstretched arms. Boluses of mucus-trapped food are passed to
1210-400: A larval stage that feeds on plankton before turning into an adult worm. The Pterobranch genus most extensively studied is Rhabdopleura from Plymouth, England and from Bermuda. The following details the development of two popularly studied species of the hemichordata phylum Saccoglossus kowalevskii and Ptychodera flava . Saccoglossus kowalevskii is a direct developer and Ptychodera flava
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#17327830419741320-620: A little in front of the midpoint. The two halves each regenerate their missing organs over a period of several months, but the missing genital organs are often very slow to develop. The larvae of some echinoderms are capable of asexual reproduction. This has long been known to occur among starfish and brittle stars, but has more recently been observed in a sea cucumber, a sand dollar and a sea urchin. This may be by autotomising parts that develop into secondary larvae, by budding , or by splitting transversely . Autotomised parts or buds may develop directly into fully formed larvae, or may pass through
1430-478: A main line of defence against potential pathogens. Depending on the class, echinoderms may have spherule cells (for cytotoxicity, inflammation, and anti-bacterial activity), vibratile cells (for coelomic fluid movement and clotting), and crystal cells (which may serve for osmoregulation in sea cucumbers). The coelomocytes secrete antimicrobial peptides against bacteria, and have a set of lectins and complement proteins as part of an innate immune system that
1540-520: A moderate diversity of embryological development among these species. Hemichordates are classically known to develop in two ways, both directly and indirectly. Hemichordates are a phylum composed of two classes, the enteropneusts and the pterobranchs, both being forms of marine worm. The enteropneusts have two developmental strategies: direct and indirect development. The indirect developmental strategy includes an extended pelagic plankotrophic tornaria larval stage, which means that this hemichordate exists in
1650-402: A muscular and ciliated cephalic shield used in locomotion and in secreting the coenecium. The mesosome extends into one pair (in the genus Rhabdopleura ) or several pairs (in the genus Cephalodiscus ) of tentaculated arms used in filter feeding. The metasome, or trunk, contains a looped digestive tract, gonads, and extends into a contractile stalk that connects individuals to the other members of
1760-514: A netrin that groups with netrin gene class 1 and 2. Netrin is important in patterning of the neural system in chordates, as well as is the molecule Shh, but S. kowalevskii was only found to have one hh gene and it appears to be expressed in a region that is uncommon to where it is usually expressed in developing chordates along the ventral midline. Hemichordata are divided into two classes: the Enteropneusta , commonly called acorn worms, and
1870-400: A phylum based on body plan has been proposed by paleontologists Graham Budd and Sören Jensen (as Haeckel had done a century earlier). The definition was posited because extinct organisms are hardest to classify: they can be offshoots that diverged from a phylum's line before the characters that define the modern phylum were all acquired. By Budd and Jensen's definition, a phylum is defined by
1980-471: A phylum much more diverse than it would be otherwise. Total numbers are estimates; figures from different authors vary wildly, not least because some are based on described species, some on extrapolations to numbers of undescribed species. For instance, around 25,000–27,000 species of nematodes have been described, while published estimates of the total number of nematode species include 10,000–20,000; 500,000; 10 million; and 100 million. The kingdom Plantae
2090-706: A phylum, other phylum-level ranks appear, such as the case of Bacillariophyta (diatoms) within Ochrophyta . These differences became irrelevant after the adoption of a cladistic approach by the ISP, where taxonomic ranks are excluded from the classifications after being considered superfluous and unstable. Many authors prefer this usage, which lead to the Chromista-Protozoa scheme becoming obsolete. Currently there are 40 bacterial phyla (not including " Cyanobacteria ") that have been validly published according to
2200-430: A pore (or a pair of pores in sea urchins) to the exterior, forming a podium or tube foot . The water vascular system assists with the distribution of nutrients throughout the animal's body; it is most visible in the tube feet which can be extended or contracted by the redistribution of fluid between the foot and the internal ampulla. The organisation of the water vascular system is somewhat different in ophiuroids, where
2310-449: 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
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#17327830419742420-541: A sea urchin is overturned, it can extend its tube feet in one ambulacral area far enough to bring them within reach of the substrate and then successively attach feet from the adjoining area until it is righted. Some species bore into rock, usually by grinding away at the surface with their mouthparts. Sea cucumbers are generally sluggish animals. Many can move on the surface of the seabed or burrow through sand or mud using peristaltic movements; some have short tube feet on their under surface with which they can creep along in
2530-401: A set of characters shared by all its living representatives. This approach brings some small problems—for instance, ancestral characters common to most members of a phylum may have been lost by some members. Also, this definition is based on an arbitrary point of time: the present. However, as it is character based, it is easy to apply to the fossil record. A greater problem is that it relies on
2640-450: A similar order and completes a 64 cell stage, finally the seventh cleavage marks the end of the cleavage stage with a blastula with 128 blastomeres. This structure goes on to go through gastrulation movements which will determine the body plan of the resulting gill slit larva, this larva will ultimately give rise to the marine acorn worm. Much of the genetic work done on hemichordates has been done to make comparison with chordates, so many of
2750-402: 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 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
2860-401: A subjective decision about which groups of organisms should be considered as phyla. The approach is useful because it makes it easy to classify extinct organisms as " stem groups " to the phyla with which they bear the most resemblance, based only on the taxonomically important similarities. However, proving that a fossil belongs to the crown group of a phylum is difficult, as it must display
2970-538: A sudden encirclement by their flexible arms. The limbs then bend under the disc to transfer the food to the jaws and mouth. Many sea urchins feed on algae, often scraping off the thin layer of algae covering the surfaces of rocks with their specialised mouthparts known as Aristotle's lantern. Other species devour smaller organisms, which they may catch with their tube feet. They may also feed on dead fish and other animal matter. Sand dollars may perform suspension feeding and feed on phytoplankton , detritus, algal pieces and
3080-424: A term that remains in use today for groups of plants, algae and fungi. The definitions of zoological phyla have changed from their origins in the six Linnaean classes and the four embranchements of Georges Cuvier . Informally, phyla can be thought of as groupings of organisms based on general specialization of body plan . At its most basic, a phylum can be defined in two ways: as a group of organisms with
3190-459: A tip shaped like a suction pad in which a vacuum can be created by contraction of muscles. This combines with some stickiness from the secretion of mucus to provide adhesion. The tube feet contract and relax in waves which move along the adherent surface, and the animal moves slowly along. Brittle stars are the most agile of the echinoderms. Any one of the arms can form the axis of symmetry, pointing either forwards or back. The animal then moves in
3300-543: A very small number of species, the eggs are retained in the coelom where they develop viviparously , later emerging through ruptures in the body wall. In some crinoids, the embryos develop in special breeding bags, where the eggs are held until sperm released by a male happens to find them. One species of seastar , Ophidiaster granifer , reproduces asexually by parthenogenesis . In certain other asterozoans , adults reproduce asexually until they mature, then reproduce sexually. In most of these species, asexual reproduction
3410-474: Is a paraphyletic taxon, which is less acceptable to present-day biologists than in the past. Proposals have been made to divide it among several new kingdoms, such as Protozoa and Chromista in the Cavalier-Smith system . Protist taxonomy has long been unstable, with different approaches and definitions resulting in many competing classification schemes. Many of the phyla listed below are used by
Hemichordate - Misplaced Pages Continue
3520-484: Is a level of classification or taxonomic rank below kingdom and above class . Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature for algae, fungi, and plants accepts the terms as equivalent. Depending on definitions, the animal kingdom Animalia contains about 31 phyla, the plant kingdom Plantae contains about 14 phyla, and
3630-446: 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
3740-479: Is an indirect developer. Most of what has been detailed in Hemichordate development has come from hemichordates that develop directly. P. flava’s early cleavage pattern is similar to that of S. kowalevskii . The first and second cleavages from the single cell zygote of P. flava are equal cleavages, are orthogonal to each other and both include the animal and vegetal poles of the embryo. The third cleavage
3850-416: Is by transverse fission with the disc splitting in two. Both the lost disc area and the missing arms regrow, so an individual may have arms of varying lengths. During the period of regrowth, they have a few tiny arms and one large arm, and are thus often known as "comets". Adult sea cucumbers reproduce asexually by transverse fission. Holothuria parvula uses this method frequently, splitting into two
3960-438: Is characterized by a muscular organization. The anteroposterior axis is divided into three parts: the anterior prosome, the intermediate mesosome, and the posterior metasome. The body of acorn worms is worm-shaped and divided into an anterior proboscis, an intermediate collar, and a posterior trunk. The proboscis is a muscular and ciliated organ used in locomotion and in the collection and transport of food particles. The mouth
4070-405: Is defined in various ways by different biologists (see Current definitions of Plantae ). All definitions include the living embryophytes (land plants), to which may be added the two green algae divisions, Chlorophyta and Charophyta , to form the clade Viridiplantae . The table below follows the influential (though contentious) Cavalier-Smith system in equating "Plantae" with Archaeplastida ,
4180-422: Is equal and equatorial so that the embryo has four blastomeres both in the vegetal and the animal pole. The fourth division occurs mainly in blastomeres in the animal pole, which divide transversally as well as equally to make eight blastomeres. The four vegetal blastomeres divide equatorially but unequally and they give rise to four big macromeres and four smaller micromeres. Once this fourth division has occurred,
4290-496: Is especially common in cold water species where planktonic larvae might not be able to find sufficient food. These retained eggs are usually few in number and are supplied with large yolks to nourish the developing embryos. In starfish, the female may carry the eggs in special pouches, under her arms, under her arched body, or even in her cardiac stomach. Many brittle stars are hermaphrodites; they often brood their eggs, usually in special chambers on their oral surfaces, but sometimes in
4400-481: Is generally included in kingdom Fungi, though its exact relations remain uncertain, and it is considered a protozoan by the International Society of Protistologists (see Protista , below). Molecular analysis of Zygomycota has found it to be polyphyletic (its members do not share an immediate ancestor), which is considered undesirable by many biologists. Accordingly, there is a proposal to abolish
4510-422: Is known only from the larva of a single species, Planctosphaera pelagica . The class Graptolithina , formerly considered extinct, is now placed within the pterobranchs, represented by a single living genus Rhabdopleura . Acorn worms are solitary worm-shaped organisms. They generally live in burrows (the earliest secreted tubes) and are deposit feeders, but some species are pharyngeal filter feeders , while
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4620-426: 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
4730-401: Is located between the proboscis and the collar. The trunk is the longest part of the animal. It contains the pharynx, which is perforated with gill slits (or pharyngeal slits), the oesophagus, a long intestine, and a terminal anus. It also contains the gonads. A post-anal tail is present in juvenile members of the acorn worm family Harrimaniidae . The prosome of pterobranchs is specialized into
4840-520: Is located dorsally within the proboscis complex, and does not contain any blood. Instead it moves the blood indirectly by pulsating against the dorsal blood vessel. Together with the echinoderms , the hemichordates form the Ambulacraria , which are the closest extant phylogenetic relatives of chordates . Thus these marine worms are of great interest for the study of the origins of chordate development. There are several species of hemichordates, with
4950-402: 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
5060-673: Is possible that the extinct organism Etacystis is a member of the Hemichordata, either within or with close affinity to the Pterobranchia. There are 130 described species of Hemichordata and many new species are being discovered, especially in the deep sea. A phylogenetic tree showing the position of the hemichordates is: Cephalochordata [REDACTED] Tunicata [REDACTED] Vertebrata / Craniata [REDACTED] Echinodermata [REDACTED] Hemichordata [REDACTED] The internal relationships within
5170-441: Is still being characterised. Echinoderms have a simple radial nervous system that consists of a modified nerve net of interconnected neurons with no central brain , although some do possess ganglia . Nerves radiate from central rings around the mouth into each arm or along the body wall; the branches of these nerves coordinate the movements of the organism and the synchronisation of the tube feet. Starfish have sensory cells in
5280-482: Is synchronised in some species, usually with regard to the lunar cycle. In other species, individuals may aggregate during the reproductive season, increasing the likelihood of successful fertilisation. Internal fertilisation has been observed in three species of sea star, three brittle stars and a deep-water sea cucumber. Even at abyssal depths , where no light penetrates, echinoderms often synchronise their reproductive activity. Some echinoderms brood their eggs . This
5390-690: The Bacteriological Code Currently there are 2 phyla that have been validly published according to the Bacteriological Code Other phyla that have been proposed, but not validly named, include: Echinoderms 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
5500-565: The Catalogue of Life , and correspond to the Protozoa-Chromista scheme, with updates from the latest (2022) publication by Cavalier-Smith . Other phyla are used commonly by other authors, and are adapted from the system used by the International Society of Protistologists (ISP). Some of the descriptions are based on the 2019 revision of eukaryotes by the ISP. The number of protist phyla varies greatly from one classification to
5610-540: The Mesozoic Marine Revolution . The name echinoderm is from Ancient Greek ἐχῖνος ( ekhînos ) 'hedgehog' and δέρμα ( dérma ) 'skin'. The name Echinodermata 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
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#17327830419745720-599: The Pterobranchia , which includes the graptolites . A third class, Planctosphaeroidea , is proposed based on a single species known only from larvae. The phylum contains about 120 living species. Hemichordata appears to be sister to the Echinodermata as Ambulacraria; Xenoturbellida may be basal to that grouping. Pterobranchia may be derived from within Enteropneusta, making Enteropneusta paraphyletic. It
5830-540: 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 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
5940-553: The crown-of-thorns starfish are long and sharp and can cause a painful puncture wound as the epithelium covering them contains a toxin. Because of their catch connective tissue, which can change rapidly from a flaccid to a rigid state, echinoderms are very difficult to dislodge from crevices. Some sea cucumbers have a cluster of cuvierian tubules which can be ejected as long sticky threads from their anus to entangle and permanently disable an attacker. Sea cucumbers occasionally defend themselves by rupturing their body wall and discharging
6050-760: The order Apodida have a single statocyst adjoining each radial nerve, and some have an eyespot at the base of each tentacle. The gonads at least periodically occupy much of the body cavities of sea urchins and sea cucumbers, while the less voluminous crinoids, brittle stars and starfish have two gonads in each arm. While the ancestors of modern echinoderms are believed to have had one genital aperture, many organisms have multiple gonopores through which eggs or sperm may be released. Many echinoderms have great powers of regeneration . Many species routinely autotomize and regenerate arms and viscera . Sea cucumbers often discharge parts of their internal organs if they perceive themselves to be threatened, regenerating them over
6160-426: The sea-lily Comaster schlegelii has two hundred. Genetic studies have shown that genes directing anterior-most development are expressed along ambulacra in the center of starfish rays, with the next-most-anterior genes expressed in the surrounding fringe of tube feet. Genes related to the beginning of the trunk are expressed at the ray margins, but trunk genes are only expressed in interior tissue rather than on
6270-604: 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,
6380-497: The Zygomycota phylum. Its members would be divided between phylum Glomeromycota and four new subphyla incertae sedis (of uncertain placement): Entomophthoromycotina , Kickxellomycotina , Mucoromycotina , and Zoopagomycotina . Kingdom Protista (or Protoctista) is included in the traditional five- or six-kingdom model, where it can be defined as containing all eukaryotes that are not plants, animals, or fungi. Protista
6490-458: The above definitions is the "certain degree" that defines how different organisms need to be members of different phyla. The minimal requirement is that all organisms in a phylum should be clearly more closely related to one another than to any other group. Even this is problematic because the requirement depends on knowledge of organisms' relationships: as more data become available, particularly from molecular studies, we are better able to determine
6600-455: The animal to the vegetal pole and usually is equal though very often can also be unequal. The second cleavage to reach the embryos four cell stage also occurs from the animal to the vegetal pole in an approximately equal fashion though like the first cleavage it’s possible to have an unequal division. The eight cell stage cleavage is latitudinal; so that each cell from the four cell stage goes on to make two cells. The fourth division occurs first in
6710-421: 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
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#17327830419746820-597: The anus located in the centre of the aboral body surface. With a few exceptions, the members of the order Paxillosida do not possess an anus. In many species of starfish, the large cardiac stomach can be everted to digest food outside the body. Some other species are able to ingest whole food items such as molluscs . Brittle stars, which have varying diets, have a blind gut with no intestine or anus; they expel food waste through their mouth. Sea urchins are herbivores and use their specialised mouthparts to graze, tear and chew their food, mainly algae . They have an oesophagus,
6930-537: The arms of sea stars, brittle stars and crinoids. The ossicles may bear external projections in the form of spines, granules or warts and they are supported by a tough epidermis . Skeletal elements are sometimes deployed in specialized ways, such as the chewing organ called " Aristotle's lantern " in sea urchins, the supportive stalks of crinoids, and the structural "lime ring" of sea cucumbers. Although individual ossicles are robust and fossilize readily, complete skeletons of starfish, brittle stars and crinoids are rare in
7040-485: The bacterial layer surrounding grains of sand. Sea cucumbers are often mobile deposit or suspension feeders, using their buccal podia to actively capture food and then stuffing the particles individually into their buccal cavities. Others ingest large quantities of sediment, absorb the organic matter and pass the indigestible mineral particles through their guts. In this way they disturb and process large volumes of substrate, often leaving characteristic ridges of sediment on
7150-448: The body surface. This means that a starfish body can more-or-less be considered to consist only of a head. Echinoderms have a mesodermal skeleton in the dermis, composed of calcite -based plates known as ossicles . If solid, these would form a heavy skeleton, so they have a sponge-like porous structure known as stereom. Ossicles may be fused together, as in the test of sea urchins, or may articulate to form flexible joints as in
7260-489: The brittle stars, six-armed species such as Ophiothela danae , Ophiactis savignyi , and Ophionotus hexactis exist, and Ophiacantha vivipara often has more than six. Echinoderms have secondary radial symmetry in portions of their body at some stage of life, most likely an adaptation to a sessile or slow-moving existence. Many crinoids and some seastars are symmetrical in multiples of the basic five; starfish such as Labidiaster annulatus possess up to fifty arms, while
7370-400: The cells of the animal pole, which end up making eight blastomeres (mesomeres) that are not radially symmetric, then the four vegetal pole blastomeres divide to make a level of four large blastomeres (macromeres) and four very small blastomeres (micromeres). The fifth cleavage occurs first in the animal cells and then in the vegetal cells to give a 32 cell blastomere. The sixth cleavage occurs in
7480-496: The chordate notochord , but this is most likely the result of convergent evolution rather than a homology . A hollow neural tube exists among some species (at least in early life), probably a primitive trait that they share with the common ancestor of chordata and the rest of the deuterostomes. Hemichordates have a nerve net and longitudinal nerves, but no brain. Some species biomineralize in calcium carbonate. Hemichordates have an open circulatory system . The heart vesicle
7590-424: The colony, produced by asexual budding. In the genus Cephalodiscus , asexually produced individuals stay attached to the contractile stalk of the parent individual until completing their development. In the genus Rhabdopleura , zooids are permanently connected to the rest of the colony via a common stolon system. They have a diverticulum of the foregut called a stomochord , previously thought to be related to
7700-417: The course of several months. Sea urchins constantly replace spines lost through damage, while sea stars and sea lilies readily lose and regenerate their arms. In most cases, a single severed arm cannot grow into a new starfish in the absence of at least part of the disc. However, in a few species a single arm can survive and develop into a complete individual, and arms are sometimes intentionally detached for
7810-399: The ecological roles of adults are the grazing of sea urchins, the sediment processing of heart urchins, and the suspension and deposit feeding of crinoids and sea cucumbers. Some sea urchins can bore into solid rock, destabilising rock faces and releasing nutrients into the ocean. Coral reefs are also bored into in this way, but the rate of accretion of carbonate material is often greater than
7920-420: The ectodermal side of the embryo, and as gastrulation progresses its expression is narrowed down to the dorsal midline but is not expressed in the post anal tail. The bmp antagonist chordin is also expressed in the endoderm of gastrulating S. kowalevskii . Besides these well known dorsalizing factors, further molecules known to be involved in dorsal ventral patterning are also present in S. kowalevskii , such as
8030-407: The embryo has reached a 16 cell stage. P. flava has a 16 cell embryo with four vegetal micromeres, eight animal mesomeres and 4 larger macromeres. Further divisions occur until P. flava finishes the blastula stage and goes on to gastrulation . The animal mesomeres of P. flava go on to give rise to the larva’s ectoderm , animal blastomeres also appear to give rise to these structures though
8140-451: 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
8250-599: The exact contribution varies from embryo to embryo. The macromeres give rise to the posterior larval ectoderm and the vegetal micromeres give rise to the internal endomesodermal tissues. Studies done on the potential of the embryo at different stages have shown that at both the two and four cell stage of development P. flava blastomeres can go on to give rise to a tornaria larvae, so fates of these embryonic cells don’t seem to be established till after this stage. Eggs of S. kowalevskii are oval in shape and become spherical in shape after fertilization. The first cleavage occurs from
8360-466: The family are free living detritivores . Many are well known for their production and accumulation of various halogenated phenols and pyrroles . Pterobranchs are filter-feeders, mostly colonial, living in a collagenous tubular structure called a coenecium . The discovery of the stem group hemichordate Gyaltsenglossus shows that early hemichordates combined aspects of the two morphologically disparate classes. The body plan of hemichordates
8470-507: The first publication of the APG system in 1998, which proposed a classification of angiosperms up to the level of orders , many sources have preferred to treat ranks higher than orders as informal clades. Where formal ranks have been provided, the traditional divisions listed below have been reduced to a very much lower level, e.g. subclasses . Wolf plants Hepatophyta Liver plants Coniferophyta Cone-bearing plant Phylum Microsporidia
8580-447: The form of ammonia , diffuses out through the respiratory surfaces. The coelomic fluid contains the coelomocytes , or immune cells. There are several types of immune cells, which vary among classes and species. All classes possess a type of phagocytic amebocyte, which engulf invading particles and infected cells, aggregate or clot, and may be involved in cytotoxicity . These cells are usually large and granular, and are believed to be
8690-441: The fossil record. On the other hand, sea urchins are often well preserved in chalk beds or limestone. During fossilization, the cavities in the stereom are filled in with calcite that is continuous with the surrounding rock. On fracturing such rock, paleontologists can observe distinctive cleavage patterns and sometimes even the intricate internal and external structure of the test. The epidermis contains pigment cells that provide
8800-583: The fungus kingdom Fungi contains about 8 phyla. Current research in phylogenetics is uncovering the relationships among phyla within larger clades like Ecdysozoa and Embryophyta . The term phylum was coined in 1866 by Ernst Haeckel from the Greek phylon ( φῦλον , "race, stock"), related to phyle ( φυλή , "tribe, clan"). Haeckel noted that species constantly evolved into new species that seemed to retain few consistent features among themselves and therefore few features that distinguished them as
8910-401: The genetic markers identified in this group are also found in chordates or are homologous to chordates in some way. Studies of this nature have been done particularly on S. kowalevskii , and like chordates S. kowalevskii has dorsalizing bmp-like factors such as bmp 2/4 , which is homologous to Drosophila ’s decapentaplegic dpp. The expression of bmp2/4 begins at the onset of gastrulation on
9020-416: The great quantity of eggs and larva that they produce form part of the zooplankton , consumed by many marine creatures. Crinoids, on the other hand, are relatively free from predation. Antipredator defences include the presence of spines, toxins (inherent or delivered through the tube feet), and the discharge of sticky entangling threads by sea cucumbers. Although most echinoderm spines are blunt, those of
9130-399: The gut and internal organs. Starfish and brittle stars may undergo autotomy when attacked, detaching an arm; this may distract the predator for long enough for the animal to escape. Some starfish species can swim away from danger. Echinoderms are numerous invertebrates whose adults play an important role in benthic ecosystems , while the larvae are a major component of the plankton. Among
9240-437: The hemichordates are shown below. The tree is based on 16S +18S rRNA sequence data and phylogenomic studies from multiple sources. Stereobalanus Harrimaniidae Spengeliidae Torquaratoridae Ptychoderidae [REDACTED] Cephalodiscida [REDACTED] Rhabdopleurida [REDACTED] † Dendroidea † Graptoloidea Phylum In biology , a phylum ( / ˈ f aɪ l əm / ; pl. : phyla )
9350-517: 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
9460-457: 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
9570-774: The majority of starfish are active hunters. Crinoids catch food particles using the tube feet on their outspread pinnules, move them into the ambulacral grooves, wrap them in mucus, and convey them to the mouth using the cilia lining the grooves. The exact dietary requirements of crinoids have been little researched, but in the laboratory, they can be fed with diatoms. Basket stars are suspension feeders, raising their branched arms to collect zooplankton , while other brittle stars use several methods of feeding. Some are suspension feeders, securing food particles with mucus strands, spines or tube feet on their raised arms. Others are scavengers and detritus feeders. Others again are voracious carnivores and able to lasso their waterborne prey with
9680-478: The manner of a starfish. Some species drag themselves along using their buccal tentacles, while others manage to swim with peristaltic movements or rhythmic flexing. Many live in cracks, hollows and burrows and hardly move at all. Some deep-water species are pelagic and can float in the water with webbed papillae forming sails or fins. The majority of crinoids are motile, but sea lilies are sessile and attached to hard substrates by stalks. Movement in most sea lilies
9790-536: The mouth, which is linked to the anus by a loop consisting of a short oesophagus and longer intestine. The coelomic cavities of echinoderms are complex. Aside from the water vascular system, echinoderms have a haemal coelom , a peri visceral coelom, a gonadal coelom and often also a perihaemal coelom. During development, echinoderm coelom is divided into the metacoel, mesocoel and protocoel (also called somatocoel, hydrocoel and axocoel, respectively). The water vascular system, haemal system and perihaemal system form
9900-536: The movement and remodelling of existing tissues to replace lost parts. Direct transdifferentiation of one type of tissue to another during tissue replacement is also observed. Echinoderms become sexually mature after approximately two to three years, depending on the species and the environmental conditions. Almost all species have separate male and female sexes , though some are hermaphroditic . The eggs and sperm cells are typically released into open water, where fertilisation takes place. The release of sperm and eggs
10010-605: 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 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
10120-411: The next. The Catalogue of Life includes Rhodophyta and Glaucophyta in kingdom Plantae, but other systems consider these phyla part of Protista. In addition, less popular classification schemes unite Ochrophyta and Pseudofungi under one phylum, Gyrista , and all alveolates except ciliates in one phylum Myzozoa , later lowered in rank and included in a paraphyletic phylum Miozoa . Even within
10230-466: 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
10340-531: 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
10450-539: The other hand, the highly parasitic phylum Mesozoa was divided into two phyla ( Orthonectida and Rhombozoa ) when it was discovered the Orthonectida are probably deuterostomes and the Rhombozoa protostomes . This changeability of phyla has led some biologists to call for the concept of a phylum to be abandoned in favour of placing taxa in clades without any formal ranking of group size. A definition of
10560-424: The ovary or coelom. In these starfish and brittle stars, development is usually direct to the adult form, without passing through a bilateral larval stage. A few sea urchins and one species of sand dollar carry their eggs in cavities, or near their anus, holding them in place with their spines. Some sea cucumbers use their buccal tentacles to transfer their eggs to their underside or back, where they are retained. In
10670-417: 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:
10780-425: The presence of predators. Asexual reproduction produces many smaller larvae that escape better from planktivorous fish, implying that the mechanism may be an anti-predator adaptation. Development begins with a bilaterally symmetrical embryo, with a coeloblastula developing first. Gastrulation marks the opening of the "second mouth" that places echinoderms within the deuterostomes, and the mesoderm, which will host
10890-459: The purpose of asexual reproduction . During periods when they have lost their digestive tracts, sea cucumbers live off stored nutrients and absorb dissolved organic matter directly from the water. The regeneration of lost parts involves both epimorphosis and morphallaxis . In epimorphosis stem cells—either from a reserve pool or those produced by dedifferentiation —form a blastema and generate new tissues. Morphallactic regeneration involves
11000-475: The relationships between groups. So phyla can be merged or split if it becomes apparent that they are related to one another or not. For example, the bearded worms were described as a new phylum (the Pogonophora) in the middle of the 20th century, but molecular work almost half a century later found them to be a group of annelids , so the phyla were merged (the bearded worms are now an annelid family ). On
11110-449: The seabed to undergo metamorphosis, and the larval arms and gut degenerate. The left-hand side of the larva develops into the oral surface of the juvenile, while the right side becomes the aboral surface. At this stage the pentaradial symmetry develops. A plankton-eating larva, living and feeding in the water column, is considered to be the ancestral larval type for echinoderms, but in extant echinoderms, some 68% of species develop using
11220-470: The seabed, raising their body with the help of their arms, or swim using their arms. Most species of sea feather, however, are largely sedentary, seldom moving far from their chosen place of concealment. The modes of feeding vary greatly between the different echinoderm taxa. Crinoids and some brittle stars tend to be passive filter-feeders, enmeshing suspended particles from passing water. Most sea urchins are grazers; sea cucumbers are deposit feeders; and
11330-441: The seabed. Some sea cucumbers live infaunally in burrows, anterior-end down and anus on the surface, swallowing sediment and passing it through their gut. Other burrowers live anterior-end up and wait for detritus to fall into the entrances of the burrows or rake in debris from the surface nearby with their buccal podia. Nearly all starfish are detritus feeders or carnivores, though a few are suspension feeders. Small fish landing on
11440-412: 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 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
11550-416: The skeleton, migrates inwards. The secondary body cavity, the coelom, forms by the partitioning of three body cavities. The larvae are often planktonic , but in some species the eggs are retained inside the female, while in some the female broods the larvae. The larvae pass through several stages, which have specific names derived from the taxonomic names of the adults or from their appearance. For example,
11660-649: The soft body parts. As the adductor muscle of the bivalve relaxes, more stomach is inserted and when digestion is complete, the stomach is returned to its usual position in the starfish with its now liquefied bivalve meal inside it. Other starfish evert the stomach to feed on sponges, sea anemones, corals, detritus and algal films. Despite their low nutrition value and the abundance of indigestible calcite, echinoderms are preyed upon by many organisms, including bony fish , sharks , eider ducks , gulls , crabs , gastropod molluscs , other echinoderms, sea otters , Arctic foxes and humans. Larger starfish prey on smaller ones;
11770-411: The tube feet are coordinated, as the animal glides steadily along. Some burrowing starfish have points rather than suckers on their tube feet and they are able to "glide" across the seabed at a faster rate. Sea urchins use their tube feet to move around in a similar way to starfish. Some also use their articulated spines to push or lever themselves along or lift their oral surfaces off the substrate. If
11880-430: The tube feet lack suckers and are used in a back-and-forth wafting motion to pass food particles captured by the arms towards the central mouth. In the asteroids, the same motion is employed to move the animal across the ground. Echinoderms possess a simple digestive system which varies according to the animal's diet. Starfish are mostly carnivorous and have a mouth, oesophagus, two-part stomach, intestine and rectum, with
11990-403: The tubular coelomic system. Echinoderms are unusual in having both a coelomic circulatory system (the water vascular system) and a haemal circulatory system, as most groups of animals have just one of the two. Haemal and perihaemal systems are derived from the original coelom, forming an open and reduced circulatory system. This usually consists of a central ring and five radial vessels. There
12100-447: 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
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