Brachiopods, phylum Brachiopoda, are a group of lophotrochozoan animals that have hard "valves" on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection. Two major groups are recognized and inarticulate; the word "articulate" is used to describe the tooth-and-groove features of the valve-hinge, present in the articulate group, absent from the inarticulate group. This is the leading diagnostic feature, by which the two main groups can be distinguished. Articulate brachiopods have toothed hinges and simple opening and closing muscles, while inarticulate brachiopods have untoothed hinges and a more complex system of muscles used to keep the two valves aligned. In a typical brachiopod a stalk-like pedicle projects from an opening in one of the valves near the hinges, known as the pedicle valve, keeping the animal anchored to the seabed but clear of silt that would obstruct the opening.
The word "brachiopod" is formed from podos. They are known as "lamp shells", since the curved shells of the class Terebratulida look rather like pottery oil-lamps. Lifespans range from three to over thirty years. Ripe gametes float from the gonads into the main coelom and exit into the mantle cavity; the larvae of inarticulate brachiopods are miniature adults, with lophophores that enable the larvae to feed and swim for months until the animals become heavy enough to settle to the seabed. The planktonic larvae of articulate species do not resemble the adults, but rather look like blobs with yolk sacs, remain among the plankton for only a few days before leaving the water column upon metamorphosing. In addition to the traditional classification of brachiopods into inarticulate and articulate, two approaches appeared in the 1990s: one approach groups the inarticulate Craniida with articulate brachiopods, since both use the same material in the mineral layers of their shell. However, some taxonomists believe it is premature to suggest higher levels of classification such as order and recommend a bottom-up approach that identifies genera and groups these into intermediate groups.
Traditionally, brachiopods have been regarded as members of, or as a sister group to, the deuterostomes, a superphylum that includes chordates and echinoderms. One type of analysis of the evolutionary relationships of brachiopods has always placed brachiopods as protostomes while another type has split between placing brachiopods among the protostomes or the deuterostomes, it was suggested in 2003 that brachiopods had evolved from an ancestor similar to Halkieria, a slug-like Cambrian animal with "chain mail" on its back and a shell at the front and rear end. However, new fossils found in 2007 and 2008 showed that the "chain mail" of tommotiids formed the tube of a sessile animal. Lineages of brachiopods that have both fossil and extant taxa appeared in the early Cambrian and Carboniferous periods, respectively. Other lineages have arisen and become extinct, sometimes during severe mass extinctions. At their peak in the Paleozoic era, the brachiopods were among the most abundant filter-feeders and reef-builders, occupied other ecological niches, including swimming in the jet-propulsion style of scallops.
Brachiopod fossils have been useful indicators of climate changes during the Paleozoic. However, after the Permian–Triassic extinction event, brachiopods recovered only a third of their former diversity. A study in 2007 concluded the brachiopods were vulnerable to the Permian–Triassic extinction, as they built calcareous hard parts and had low metabolic rates and weak respiratory systems, it was thought that brachiopods went into decline after the Permian–Triassic extinction, were out-competed by bivalves, but a study in 1980 found both brachiopod and bivalve species increased from the Paleozoic to modern times, with bivalves increasing faster. Brachiopods live only in the sea, most species avoid locations with strong currents or waves; the larvae of articulate species settle in and form dense populations in well-defined areas while the larvae of inarticulate species swim for up to a month and have wide ranges. Brachiopods now live in cold water and low light. Fish and crustaceans seem to find brachiopod flesh distasteful and attack them.
Among brachiopods, only the lingulids have been fished commercially, on a small scale. One brachiopod species may be a measure of environmental conditions around an oil terminal being built in Russia on the shore of the Sea of Japan. Modern brachiopods range from 1 to 100 millimetres long, most species are about 10 to 30 millimetres; the largest brachiopods known – Gigantoproductus and Titanaria, reaching 30 to 38 centimetres in width – occurred in the upper part of the Lower Carboniferous. Each has two valves which cover the dorsal and ventral surface of the animal, unlike bivalve molluscs whose shells cover the lateral surfaces; the valves are t
Triops is a genus of small crustaceans in the order Notostraca. Some species are considered living fossils, with a fossil record that reaches back to the late Carboniferous, 300 million years ago; the long lasting resting eggs of one species, Triops longicaudatus, are sold in kits as a pet. In contact with fresh water the animals hatch. Most Triops have a life expectancy of up to 90 days, can tolerate a pH range of 6–10. In nature they inhabit temporary pools; the genus Triops can be distinguished from the only other genus of Notostraca, Lepidurus, by the form of the telson, which bears only a pair of long, thin caudal extensions in Triops, while Lepidurus bears a central platelike process. Only 24 hours after hatching they resemble miniature versions of the adult form. Triops are sometimes called "living fossils," since fossils attributable to this genus have been found in rocks of Carboniferous age, an estimated 300 million years ago, one extant species, Triops cancriformis, has hardly changed since the Jurassic period.
Triops can be found in Africa, Asia, South America, in some parts of North America where the climate is right. Some eggs stay hatch when rain soaks the area. Triops are found in vernal pools. Most species reproduce sexually, but some populations are dominated by hermaphrodites which produce internally fertilised eggs. Reproduction in T. cancriformis varies with latitude, with sexual reproduction dominating in the south of its range, parthenogenesis dominating in the north. Triops eggs enter a state of extended diapause when dry, will tolerate temperatures of up to 98 °C for 16 hours, whereas the adult cannot survive temperatures above 34 °C for 24 hours or 40 °C for 2 hours; the diapause prevents the eggs from hatching too soon after rain. The name Triops comes from the Greek τρία meaning "three" and ὤψ meaning "eye"; the head of T. longicaudatus bears a pair of dorsal compound eyes that lie close to each other and are nearly fused together. The compound eyes are sessile. In addition, there is a naupliar ocellus between them.
The compound eyes are on the surface of the head. All the eyes, are visible through the shell covering of the head. Franz von Paula Schrank was the first author to use the genus name Triops, coining it in his 1803 work on the fauna of Bavaria, their German name was Dreyauge, which means'three-eye'. He collected and described specimens from the same locality in Regensburg from which Schäffer, another naturalist who had studied the Notostraca, obtained his specimens in the 1750s. However, other authors, starting with Louis Augustin Guillaume Bosc, had adopted the genus name Apus for the organisms Schrank had named Triops Ludwig Keilhack used the genus name Triops in his 1909 field identification key of the freshwater fauna of Germany, he suggested that the genus name Apus be replaced by Triops Schrank since an avian genus had been described by Giovanni Antonio Scopoli under the name Apus. However, Robert Gurney preferred the name Apus Schäffer, he suggested that the name'…Triops Schrank, may be returned to the obscurity from which it was unearthed'.
This controversy was not resolved until the 1950s. In his 1955 taxonomic review of the Notostraca, Alan R. Longhurst supported Keilhack's genus name Triops over Apus. Longhurst provided historical evidence to support this position; the International Commission on Zoological Nomenclature followed Longhurst in their 1958 ruling on the usage and origin of the genus names Triops and Apus. They rejected the genus name Apus and instead recognized the genus name Triops Schrank, 1803. Although the taxonomy of the genus has not been reviewed since 1955, the following species are recognised: Triops australiensis Triops baeticus Korn, 2010 Triops cancriformis Triops emeritensis Korn & Pérez-Bote, 2010 Triops gadensis Korn & García-de-Lomas, 2010 Triops granarius Triops longicaudatus Triops mauritanicus Ghigi, 1921 Triops newberryi Thomas, 1921 Triops vicentinus Korn, Cristo & Cancela da Fonseca, 2010T. Mauritanicus was considered a subspecies of T. cancriformis by Longhurst in 1955, but was given full species status again by Korn et al. in 2006.
Note that for several of these species there are different varieties, some of which have been suggested as subspecies and separate species. T. longicaudatus, for example, may be several species lumped together, T. cancriformis is recognized as having three subspecies: T. cancriformis cancriformis, T. c. mauretanicus, T. c. simplex. The albino form has the special name of T. cancriformis var. Beni-Kabuto Ebi; the species is considered a human ally against the West Nile virus, as the individuals consume Culex mosquito larvae. They are used as a biological agent in Japan, eating weeds in rice paddies; the Beni-Kabuto Ebi Albino variant of T. cancriformis is valued for this purpose. In Wyoming, the presence of T. longicaudatus indicates a good chance of the hatching of spadefoot frogs. Dried eggs of T. longicaudatus are sold in kits to be raised as aquarium pets, sold under the name of "aquasaurs", "trigons" or "triops". Among enthusiasts, T. cancriformis is common. Other species encountered in captivity include T. australiensis, T. newberryi and T. granarius.
The red forms of T. longicaudatus and T. ca
Vestigiality is the retention during the process of evolution of genetically determined structures or attributes that have lost some or all of their ancestral function in a given species. Assessment of the vestigiality must rely on comparison with homologous features in related species; the emergence of vestigiality occurs by normal evolutionary processes by loss of function of a feature, no longer subject to positive selection pressures when it loses its value in a changing environment. The feature may be selected against more urgently when its function becomes definitively harmful, but if the lack of the feature provides no advantage, its presence provides no disadvantage, the feature may not be phased out by natural selection and persist across species. Examples of vestigial structures are the loss of functional wings in island-dwelling birds. Vestigial features may take various forms. Like most other physical features, however functional, vestigial features in a given species may successively appear and persist or disappear at various stages within the life cycle of the organism, ranging from early embryonic development to late adulthood.
Vestigiality, biologically speaking, refers to organisms retaining organs that have lost their original function. The issue is controversial and not without dispute. In addition, the term vestigiality is useful in referring to many genetically determined features, either morphological, behavioral, or physiological. A classic example at the level of gross anatomy is the human vermiform appendix—though vestigial in the sense of retaining no significant digestive function, the appendix still has immunological roles and is useful in maintaining gut flora. Similar concepts apply at the molecular level—some nucleic acid sequences in eukaryotic genomes have no known biological function; the simple fact that it is noncoding DNA does not establish. Furthermore if an extant DNA sequence is functionless, it does not follow that it has descended from an ancestral sequence of functional DNA. Logically such DNA would not be vestigial in the sense of being the vestige of a functional structure. In contrast pseudogenes have lost their protein-coding ability or are otherwise no longer expressed in the cell.
Whether they have any extant function or not, they have lost their former function and in that sense they do fit the definition of vestigiality. Vestigial structures are called vestigial organs, although many of them are not organs; such vestigial structures are degenerate, atrophied, or rudimentary, tend to be much more variable than homologous non-vestigial parts. Although structures regarded "vestigial" may have lost some or all of the functional roles that they had played in ancestral organisms, such structures may retain lesser functions or may have become adapted to new roles in extant populations, it is important to avoid confusion of the concept of vestigiality with that of exaptation. Both may occur together depending on the relevant point of view. In exaptation a structure used for one purpose is modified for a new one. For example, the wings of penguins would be exaptational in the sense of serving a substantial new purpose, but might still be regarded as vestigial in the sense of having lost the function of flight.
In contrast Darwin argued that the wings of emus would be vestigial, as they appear to have no major extant function. The ostrich uses its wings in displays and temperature control, though they are undoubtedly vestigial as structures for flight. Vestigial characters range from detrimental through neutral to favorable in terms of selection; some may be of some limited utility to an organism but still degenerate over time if they do not confer a significant enough advantage in terms of fitness to avoid the effects of genetic drift or competing selective pressures. Vestigiality in its various forms presents many examples of evidence for biological evolution. Vestigial structures have been noticed since ancient times, the reason for their existence was long speculated upon before Darwinian evolution provided a accepted explanation. In the 4th century BC, Aristotle was one of the earliest writers to comment, in his History of Animals, on the vestigial eyes of moles, calling them "stunted in development" due to the fact that moles can scarcely see.
However, only in recent centuries have anatomical vestiges become a subject of serious study. In 1798, Étienne Geoffroy Saint-Hilaire noted on vestigial structures: His colleague, Jean-Baptiste Lamarck, named a number of vestigial structures in his 1809 book Philosophie Zoologique. Lamarck noted "Olivier's Spalax, which lives underground like the mole, is exposed to daylight less than the mole, has altogether lost the use of sight: so that it shows nothing more than vestiges of this organ."Charles Darwin was familiar with the concept of vestigial structures, though the term for them did not yet exist. He listed a number of them in The Descent of Man, including the muscles of the ear, wisdom teeth, the appe
The Ancient Greek language includes the forms of Greek used in Ancient Greece and the ancient world from around the 9th century BCE to the 6th century CE. It is roughly divided into the Archaic period, Classical period, Hellenistic period, it is succeeded by medieval Greek. Koine is regarded as a separate historical stage of its own, although in its earliest form it resembled Attic Greek and in its latest form it approaches Medieval Greek. Prior to the Koine period, Greek of the classic and earlier periods included several regional dialects. Ancient Greek was the language of Homer and of fifth-century Athenian historians and philosophers, it has contributed many words to English vocabulary and has been a standard subject of study in educational institutions of the Western world since the Renaissance. This article contains information about the Epic and Classical periods of the language. Ancient Greek was a pluricentric language, divided into many dialects; the main dialect groups are Attic and Ionic, Aeolic and Doric, many of them with several subdivisions.
Some dialects are found in standardized literary forms used in literature, while others are attested only in inscriptions. There are several historical forms. Homeric Greek is a literary form of Archaic Greek used in the epic poems, the "Iliad" and "Odyssey", in poems by other authors. Homeric Greek had significant differences in grammar and pronunciation from Classical Attic and other Classical-era dialects; the origins, early form and development of the Hellenic language family are not well understood because of a lack of contemporaneous evidence. Several theories exist about what Hellenic dialect groups may have existed between the divergence of early Greek-like speech from the common Proto-Indo-European language and the Classical period, they differ in some of the detail. The only attested dialect from this period is Mycenaean Greek, but its relationship to the historical dialects and the historical circumstances of the times imply that the overall groups existed in some form. Scholars assume that major Ancient Greek period dialect groups developed not than 1120 BCE, at the time of the Dorian invasion—and that their first appearances as precise alphabetic writing began in the 8th century BCE.
The invasion would not be "Dorian" unless the invaders had some cultural relationship to the historical Dorians. The invasion is known to have displaced population to the Attic-Ionic regions, who regarded themselves as descendants of the population displaced by or contending with the Dorians; the Greeks of this period believed there were three major divisions of all Greek people—Dorians and Ionians, each with their own defining and distinctive dialects. Allowing for their oversight of Arcadian, an obscure mountain dialect, Cypriot, far from the center of Greek scholarship, this division of people and language is quite similar to the results of modern archaeological-linguistic investigation. One standard formulation for the dialects is: West vs. non-west Greek is the strongest marked and earliest division, with non-west in subsets of Ionic-Attic and Aeolic vs. Arcadocypriot, or Aeolic and Arcado-Cypriot vs. Ionic-Attic. Non-west is called East Greek. Arcadocypriot descended more from the Mycenaean Greek of the Bronze Age.
Boeotian had come under a strong Northwest Greek influence, can in some respects be considered a transitional dialect. Thessalian had come under Northwest Greek influence, though to a lesser degree. Pamphylian Greek, spoken in a small area on the southwestern coast of Anatolia and little preserved in inscriptions, may be either a fifth major dialect group, or it is Mycenaean Greek overlaid by Doric, with a non-Greek native influence. Most of the dialect sub-groups listed above had further subdivisions equivalent to a city-state and its surrounding territory, or to an island. Doric notably had several intermediate divisions as well, into Island Doric, Southern Peloponnesus Doric, Northern Peloponnesus Doric; the Lesbian dialect was Aeolic Greek. All the groups were represented by colonies beyond Greece proper as well, these colonies developed local characteristics under the influence of settlers or neighbors speaking different Greek dialects; the dialects outside the Ionic group are known from inscriptions, notable exceptions being: fragments of the works of the poet Sappho from the island of Lesbos, in Aeolian, the poems of the Boeotian poet Pindar and other lyric poets in Doric.
After the conquests of Alexander the Great in the late 4th century BCE, a new international dialect known as Koine or Common Greek developed based on Attic Greek, but with influence from other dialects. This dialect replaced most of the older dialects, although Doric dialect has survived in the Tsakonian language, spoken in the region of modern Sparta. Doric has passed down its aorist terminations into most verbs of Demotic Greek. By about the 6th century CE, the Koine had metamorphosized into Medieval Greek. Ancient Macedonian was an Indo-European language at least related to Greek, but its exact relationship is unclear because of insufficient data: a dialect of Greek; the Macedonian dialect (or l
Clam shrimp are a taxon of bivalved branchiopod crustaceans that resemble the unrelated bivalved molluscs. They are extant, known from the fossil record, from at least the Devonian period and before, they were classified in a single order Conchostraca, which proved to be paraphyletic, being separated into three different orders: Cyclestherida and Spinicaudata. Both valves of the shell are held together by a strong closing muscle; the animals react to danger by contracting the muscle, so that the valves close and the crustacean, as if dead, lies motionlessly at the bottom of the pool. In most species the head is dorsoventrally compressed; the sessile compound eyes are close together and located on the forehead. In front of them is a simple naupliar eye; the first pair of antennae is unsegmented. The second pair of antennae, however, is biramous. Both branches are covered with numerous bristles; the crustaceans swim by swooping the antennae. In the common genus Lynceus, which can open its spherical valves wide, the thoracic legs move in an oar-like manner along with the antennae.
The number of segments constituting the thorax varies from 10 to 32, the number of legs varies accordingly. They are similar in structure to the legs of tadpole shrimp, their size decreases from front to back. In females, the outer lobes of several middle legs are modified into long, upward-bending threadlike outgrowths, used to hold the eggs on the dorsal side of the body under the shell. However, the main functions of the thoracic legs are respiration and carrying food forward to the mouth; the gills are the outer lobes of all thoracic legs that are closest to the base of the leg. The legs are in constant movement, the water between the valves of the carapace is renewed; the body ends in a large chitinised telson, either laterally compressed and bears a pair of large hooks, or dorsoventrally compressed, with short hooks. Clam shrimp have different reproductive strategies. For example, within the family Limnadiidae are found dioecious and androdioecious species The eggs are surrounded by a tough shell and can withstand drying out and other hostile conditions.
In some species these eggs can hatch after as long as 7 years. When the egg arrives in a suitable pool, a larva hatches out at the nauplius stage. Clam shrimp nauplii are distinguished by small front antennae. At the second stage, the larva develops the small shell, they develop quickly. For instance, Cyzicus reaches sexual maturity in 19 days after hatching. Extant clam shrimp belong to three orders, divided into five families. However, extinct species of these crustaceans are studied by geologists. In freshwater deposits poor in fossils, the well-preserved clam shrimp shells are found quite often, they help identify the age of the corresponding strata. During the past geological periods clam shrimp were more numerous and diverse than they are now. 300 extinct species are known, half as many living species. The oldest clam shrimp, such as Asmussia murchisoniana, were found in Devonian deposits. Many extinct species Triassic ones, lived in the sea, where no clam shrimp remain today. Introduction to the Branchiopoda Data related to Cyclestherida at Wikispecies Data related to Laevicaudata at Wikispecies Data related to Spinicaudata at Wikispecies
In biology, detritus is dead particulate organic material. It includes the bodies or fragments of dead organisms as well as fecal material. Detritus is colonized by communities of microorganisms which act to decompose the material. In terrestrial ecosystems, it is encountered as leaf litter and other organic matter intermixed with soil, denominated "soil organic matter". Detritus of aquatic ecosystems is organic material suspended in water and piling up on seabed floors, referred to as marine snow. Dead plants or animals, material derived from animal tissues lose their form, due to both physical processes and the action of decomposers, including grazers and fungi. Decomposition, the process through which organic matter is decomposed, takes place in many stages. Materials like proteins and sugars with low molecular weight are consumed and absorbed by microorganisms and organisms that feed on dead matter. Other compounds, such as complex carbohydrates are broken down more slowly; the various microorganisms involved in the decomposition break down the organic materials in order to gain the resources they require for their own survival and proliferation.
Accordingly, at the same time that the materials of plants and animals are being broken d making up the bodies of the microorganisms are built up by a process of assimilation. When microorganisms die, fine organic particles are produced, if these are eaten by small animals which feed on microorganisms, they will collect inside the intestine, change shape into large pellets of dung; as a result of this process, most of the materials from dead organisms disappears from view and is not present in any recognisable form, but is in fact present in the form of a combination of fine organic particles and the organisms using them as nutrients. This combination is detritus. In ecosystems on land, detritus is deposited on the surface of the ground, taking forms such as the humic soil beneath a layer of fallen leaves. In aquatic ecosystems, most detritus is suspended in water, settles. In particular, many different types of material are collected together by currents, much material settles in flowing areas.
Much detritus is used as a source of nutrition for animals. In particular, many bottom feeding animals living in mud flats feed in this way. In particular, since excreta are materials which other animals do not need, whatever energy value they might have, they are unbalanced as a source of nutrients, are not suitable as a source of nutrition on their own. However, there are many microorganisms; these microorganisms do not absorb nutrients from these particles, but shape their own bodies so that they can take the resources they lack from the area around them, this allows them to make use of excreta as a source of nutrients. In practical terms, the most important constituents of detritus are complex carbohydrates, which are persistent, the microorganisms which multiply using these absorb carbon from the detritus, materials such as nitrogen and phosphorus from the water in their environment to synthesise the components of their own cells. A characteristic type of food chain called the detritus cycle takes place involving detritus feeders and the microorganisms that multiply on it.
For example, mud flats are inhabited by many univalves. When these detritus feeders take in detritus with microorganisms multiplying on it, they break down and absorb the microorganisms, which are rich in proteins, excrete the detritus, complex carbohydrates, having hardly broken it down at all. At first this dung is a poor source of nutrition, so univalves pay no attention to it, but after several days, microorganisms begin to multiply on it again, its nutritional balance improves, so they eat it again. Through this process of eating the detritus many times over and harvesting the microorganisms from it, the detritus thins out, becomes fractured and becomes easier for the microorganisms to use, so the complex carbohydrates are steadily broken down and disappear over time. What is left behind by the detritivores is further broken down and recycled by decomposers, such as bacteria and fungi; this detritus cycle plays a large part in the so-called purification process, whereby organic materials carried in by rivers is broken down and disappears, an important part in the breeding and growth of marine resources.
In ecosystems on land, far more essential material is broken down as dead material passing through the detritus chain than is broken down by being eaten by animals in a living state. In both land and aquatic ecosystems, the role played by detritus is too large to ignore. In contrast to land ecosystems, dead materials and excreta in aquatic ecosystems are transported by water flow. In freshwater bodies organic material from plants can form a silt known as mulm or humus on the bottom; this material, some called undissolved organic carbon breaks down into dissolved organic carbon and can bond to heavy metal ions via chelation. It can break down into colored dissolved organic matter such as tannin, a specific form of tannic acid. In saltwater bodies, organic material breaks down and forms a marine snow that settles down to the ocean bottom. Detritus occurs in a variety of terrestrial habitats including forest and grassland. In forests the detritus is dominated by leaf and bacteria litter as measured by biomas
Animals are multicellular eukaryotic organisms that form the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, grow from a hollow sphere of cells, the blastula, during embryonic development. Over 1.5 million living animal species have been described—of which around 1 million are insects—but it has been estimated there are over 7 million animal species in total. Animals range in length from 8.5 millionths of a metre to 33.6 metres and have complex interactions with each other and their environments, forming intricate food webs. The category includes humans, but in colloquial use the term animal refers only to non-human animals; the study of non-human animals is known as zoology. Most living animal species are in the Bilateria, a clade whose members have a bilaterally symmetric body plan; the Bilateria include the protostomes—in which many groups of invertebrates are found, such as nematodes and molluscs—and the deuterostomes, containing the echinoderms and chordates.
Life forms interpreted. Many modern animal phyla became established in the fossil record as marine species during the Cambrian explosion which began around 542 million years ago. 6,331 groups of genes common to all living animals have been identified. Aristotle divided animals into those with those without. Carl Linnaeus created 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 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 the evolutionary relationships between animal taxa. Humans make use of many other animal species for food, including meat and eggs. Dogs have been used in hunting, while many aquatic animals are hunted for sport.
Non-human animals have appeared in art from the earliest times and are featured in mythology and religion. The word "animal" comes from the Latin animalis, having soul or living being; the biological definition includes all members of the kingdom Animalia. In colloquial usage, as a consequence of anthropocentrism, the term animal is sometimes used nonscientifically to refer only to non-human animals. Animals have several characteristics. Animals are eukaryotic and multicellular, unlike bacteria, which are prokaryotic, unlike protists, which are eukaryotic but unicellular. Unlike plants and algae, which produce their own nutrients animals are heterotrophic, feeding on organic material and digesting it internally. With few exceptions, animals breathe oxygen and respire aerobically. All animals are motile during at least part of their life cycle, but some animals, such as sponges, corals and barnacles become sessile; the blastula is a stage in embryonic development, unique to most 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 flexible framework upon which cells can move about and be reorganised, making the formation of complex structures possible; this may be calcified, forming structures such as shells and spicules. In contrast, the cells of other multicellular organisms are held in place by cell walls, so develop by progressive growth. Animal cells uniquely possess the cell junctions called tight junctions, gap junctions, desmosomes. With few exceptions—in particular, the sponges and placozoans—animal bodies are differentiated into tissues; these include muscles, which enable locomotion, nerve tissues, which transmit signals and coordinate the body. There is an internal digestive chamber with either one opening or two openings. Nearly all animals make use of some form of sexual reproduction, they produce haploid gametes by meiosis.
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, develop into a new sponge. In most other groups, the blastula undergoes more complicated rearrangement, it first invaginates to form 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 develops between them; these germ layers differentiate to form tissues and organs. Repeated instances of mating with a close relative during sexual reproduction leads to inbreeding depression within a population due to the increased prevalence of harmful recessive traits. Animals have evolved numerous mechanisms for avoiding close inbreeding. In some species, such as the splendid fairywren, females benefit by mating with multiple males, thus producing more offspring of higher genetic quality; some animals are capable of asexual reproduction, which results