Priapulida, sometimes referred to as penis worms, is a phylum of unsegmented marine worms. The name of the phylum relates to the Greek god of fertility, because their general shape and their extensible spiny introvert may recall the shape of a penis, they live in comparatively shallow waters up to 90 metres deep. Some species show a remarkable tolerance for hydrogen anoxia, they can be quite abundant in some areas. In an Alaskan bay as many as 85 adult individuals of Priapulus caudatus per square meter has been recorded, while the density of its larvae can be as high as 58,000 per square meter. Together with Echiura and Sipuncula, they were once placed in the taxon Gephyrea, but consistent morphological and molecular evidence supports their belonging to Ecdysozoa, which includes arthropods and nematodes. Fossil findings show that the mouth design of the stem-arthropod Pambdelurion is identical with that of priapulids, indicating that their mouth is an original trait inherited from the last common ancestor of both priapulids and arthropods if modern arthropods no longer possess it.
Among Ecdysozoa, their nearest relatives are Kinorhyncha and Loricifera, with which they constitute the Scalidophora clade named after the spines covering the introvert. They feed on slow-moving invertebrates, such as polychaete worms. Priapulid-like fossils are known at least as far back as the Middle Cambrian, they were major predators of the Cambrian period. However, crown-group priapulids cannot be recognized until the Carboniferous. About 20 extant species of priapulid worms are known, half of them being of meiobenthic size. Priapulids are cylindrical worm-like animals, ranging from 0.2–0.3 to 39 centimetres long, with a median anterior mouth quite devoid of any armature or tentacles. The body is divided into a main trunk or abdomen and a somewhat swollen proboscis region ornamented with longitudinal ridges; the body is ringed and has circles of spines, which are continued into the protrusible pharynx. Some species may have a tail or a pair of caudal appendages; the body has a chitinous cuticle, moulted as the animal grows.
There is a wide body-cavity, which has no connection with the renal or reproductive organs, so it is not a coelom. There are no vascular or respiratory systems, but the body cavity does contain phagocytic amoebocytes and cells containing the respiratory pigment haemerythrin; the alimentary canal is straight, consisting of an eversible pharynx, an intestine, a short rectum. The pharynx is lined by teeth; the anus is terminal, although in Priapulus one or two hollow ventral diverticula of the body-wall stretch out behind it. The nervous system consists of a nerve ring around the pharynx and a prominent cord running the length of the body with ganglia and longitudinal and transversal neurites consistent with an orthogonal organisation; the nervous system retains a basiepidermal configuration with a connection with the ectoderm, forming part of the body wall. There are no specialized sense organs, but there are sensory nerve endings in the body on the proboscis; the priapulids are gonochoristic, having two separate sexes Their male and female organs are associated with the excretory protonephridia.
They comprise a pair of branching tufts, each of which opens to the exterior on one side of the anus. The tips of these tufts enclose a flame-cell like those found in flatworms and other animals, these function as excretory organs; as the animals mature, diverticula arise on the tubes of these organs, which develop either spermatozoa or ova. These sex cells pass out through the ducts; the perigenital area of the genus Tubiluchus exhibit sexual dimorphism. Priapulid development has been reappraised because early studies reported abnormal development caused by high temperature of embryo culture. For the species Priapulus caudatus, the 80 µm egg undergoes a total and radial cleavage following a symmetrical and subequal pattern. Development is remarkably slow, with the first cleavage taking place 15 hours after fertilization, gastrulation after several days and hatching of the first'lorica' larvae after 15 to 20 days; the species Meiopriapulus fijiensis have direct development. In current systematics, they are described as protostomes, despite having a deuterostomic development.
Because the group is so ancient, it is assumed the deuterostome condition which appears to be ancestral for bilaterians have been maintained. Stem-group priapulids are known from the Middle Cambrian Burgess Shale, where their soft-part anatomy is preserved in conjunction with their gut contents – allowing a reconstruction of their diets. In addition, isolated microfossils are widespread in Cambrian deposits, allowing the distribution of priapulids – and individual species – to be tracked through Cambrian oceans. Trace fossils that are morphologically identical to modern priapulid burrows mark the start of the Cambrian period, suggesting that priapulids, or at least close anatomical relatives, evolved around this time. Crown-group priapulid body fossils are first known from the Carboniferous. Uncertain relationship "Class" PalaeoscolecidaStem-group Priapulida Class †Archaeopriapulida Family †Ottoiidae Genus †Ancalagon Genus †Fieldia Genus †Lecythioscopa Genus †Ottoia Genus †Scolecofurca Genus †Selkirkia Family †Louisellidae Genus †Anningvermis Genus †Corynetis Genus †LouisellaPhylum Priapulida Class Priapulimorpha Order Priapulimorphida Family Priapulidae G
The Carboniferous is a geologic period and system that spans 60 million years from the end of the Devonian Period 358.9 million years ago, to the beginning of the Permian Period, 298.9 Mya. The name Carboniferous means "coal-bearing" and derives from the Latin words carbō and ferō, was coined by geologists William Conybeare and William Phillips in 1822. Based on a study of the British rock succession, it was the first of the modern'system' names to be employed, reflects the fact that many coal beds were formed globally during that time; the Carboniferous is treated in North America as two geological periods, the earlier Mississippian and the Pennsylvanian. Terrestrial animal life was well established by the Carboniferous period. Amphibians were the dominant land vertebrates, of which one branch would evolve into amniotes, the first terrestrial vertebrates. Arthropods were very common, many were much larger than those of today. Vast swaths of forest covered the land, which would be laid down and become the coal beds characteristic of the Carboniferous stratigraphy evident today.
The atmospheric content of oxygen reached its highest levels in geological history during the period, 35% compared with 21% today, allowing terrestrial invertebrates to evolve to great size. The half of the period experienced glaciations, low sea level, mountain building as the continents collided to form Pangaea. A minor marine and terrestrial extinction event, the Carboniferous rainforest collapse, occurred at the end of the period, caused by climate change. In the United States the Carboniferous is broken into Mississippian and Pennsylvanian subperiods; the Mississippian is about twice as long as the Pennsylvanian, but due to the large thickness of coal-bearing deposits with Pennsylvanian ages in Europe and North America, the two subperiods were long thought to have been more or less equal in duration. In Europe the Lower Carboniferous sub-system is known as the Dinantian, comprising the Tournaisian and Visean Series, dated at 362.5-332.9 Ma, the Upper Carboniferous sub-system is known as the Silesian, comprising the Namurian and Stephanian Series, dated at 332.9-298.9 Ma.
The Silesian is contemporaneous with the late Mississippian Serpukhovian plus the Pennsylvanian. In Britain the Dinantian is traditionally known as the Carboniferous Limestone, the Namurian as the Millstone Grit, the Westphalian as the Coal Measures and Pennant Sandstone; the International Commission on Stratigraphy faunal stages from youngest to oldest, together with some of their regional subdivisions, are: A global drop in sea level at the end of the Devonian reversed early in the Carboniferous. There was a drop in south polar temperatures; these conditions had little effect in the deep tropics, where lush swamps to become coal, flourished to within 30 degrees of the northernmost glaciers. Mid-Carboniferous, a drop in sea level precipitated a major marine extinction, one that hit crinoids and ammonites hard; this sea level drop and the associated unconformity in North America separate the Mississippian subperiod from the Pennsylvanian subperiod. This happened about 323 million years ago, at the onset of the Permo-Carboniferous Glaciation.
The Carboniferous was a time of active mountain-building as the supercontinent Pangaea came together. The southern continents remained tied together in the supercontinent Gondwana, which collided with North America–Europe along the present line of eastern North America; this continental collision resulted in the Hercynian orogeny in Europe, the Alleghenian orogeny in North America. In the same time frame, much of present eastern Eurasian plate welded itself to Europe along the line of the Ural Mountains. Most of the Mesozoic supercontinent of Pangea was now assembled, although North China, South China continents were still separated from Laurasia; the Late Carboniferous Pangaea was shaped like an "O." There were two major oceans in the Carboniferous—Panthalassa and Paleo-Tethys, inside the "O" in the Carboniferous Pangaea. Other minor oceans were shrinking and closed - Rheic Ocean, the small, shallow Ural Ocean and Proto-Tethys Ocean. Average global temperatures in the Early Carboniferous Period were high: 20 °C.
However, cooling during the Middle Carboniferous reduced average global temperatures to about 12 °C. Lack of growth rings of fossilized trees suggest a lack of seasons of a tropical climate. Glaciations in Gondwana, triggered by Gondwana's southward movement, continued into the Permian and because of the lack of clear markers and breaks, the deposits of this glacial period are referred to as Permo-Carboniferous in age; the cooling and drying of the climate led to the Carboniferous Rainforest Collapse during the late Carboniferous. Tropical rainforests fragmented and were devastated by climate change. Carboniferous rocks in Europe and eastern North America consist of a repeated sequence of limestone, sandstone and coal beds. In North America, the early Carboniferous is marine
The Triassic is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.9 million years ago, to the beginning of the Jurassic Period 201.3 Mya. The Triassic is the shortest period of the Mesozoic Era. Both the start and end of the period are marked by major extinction events. Triassic began in the wake of the Permian–Triassic extinction event, which left the Earth's biosphere impoverished. Therapsids and archosaurs were the chief terrestrial vertebrates during this time. A specialized subgroup of archosaurs, called dinosaurs, first appeared in the Late Triassic but did not become dominant until the succeeding Jurassic Period; the first true mammals, themselves a specialized subgroup of therapsids evolved during this period, as well as the first flying vertebrates, the pterosaurs, like the dinosaurs, were a specialized subgroup of archosaurs. The vast supercontinent of Pangaea existed until the mid-Triassic, after which it began to rift into two separate landmasses, Laurasia to the north and Gondwana to the south.
The global climate during the Triassic was hot and dry, with deserts spanning much of Pangaea's interior. However, the climate became more humid as Pangaea began to drift apart; the end of the period was marked by yet another major mass extinction, the Triassic–Jurassic extinction event, that wiped out many groups and allowed dinosaurs to assume dominance in the Jurassic. The Triassic was named in 1834 by Friedrich von Alberti, after the three distinct rock layers that are found throughout Germany and northwestern Europe—red beds, capped by marine limestone, followed by a series of terrestrial mud- and sandstones—called the "Trias"; the Triassic is separated into Early and Late Triassic Epochs, the corresponding rocks are referred to as Lower, Middle, or Upper Triassic. The faunal stages from the youngest to oldest are: During the Triassic all the Earth's land mass was concentrated into a single supercontinent centered more or less on the equator and spanning from pole to pole, called Pangaea.
From the east, along the equator, the Tethys sea penetrated Pangaea, causing the Paleo-Tethys Ocean to be closed. In the mid-Triassic a similar sea penetrated along the equator from the west; the remaining shores were surrounded by the world-ocean known as Panthalassa. All the deep-ocean sediments laid down during the Triassic have disappeared through subduction of oceanic plates; the supercontinent Pangaea was rifting during the Triassic—especially late in that period—but had not yet separated. The first nonmarine sediments in the rift that marks the initial break-up of Pangaea, which separated New Jersey from Morocco, are of Late Triassic age. S. these thick sediments comprise the Newark Group. Because a super-continental mass has less shoreline compared to one broken up, Triassic marine deposits are globally rare, despite their prominence in Western Europe, where the Triassic was first studied. In North America, for example, marine deposits are limited to a few exposures in the west, thus Triassic stratigraphy is based on organisms that lived in lagoons and hypersaline environments, such as Estheria crustaceans.
At the beginning of the Mesozoic Era, Africa was joined with Earth's other continents in Pangaea. Africa shared the supercontinent's uniform fauna, dominated by theropods and primitive ornithischians by the close of the Triassic period. Late Triassic fossils are more common in the south than north; the time boundary separating the Permian and Triassic marks the advent of an extinction event with global impact, although African strata from this time period have not been studied. During the Triassic peneplains are thought to have formed in what is now southern Sweden. Remnants of this peneplain can be traced as a tilted summit accordance in the Swedish West Coast. In northern Norway Triassic peneplains may have been buried in sediments to be re-exposed as coastal plains called strandflats. Dating of illite clay from a strandflat of Bømlo, southern Norway, have shown that landscape there became weathered in Late Triassic times with the landscape also being shaped during that time. At Paleorrota geopark, located in Rio Grande do Sul, the Santa Maria Formation and Caturrita Formations are exposed.
In these formations, one of the earliest dinosaurs, Staurikosaurus, as well as the mammal ancestors Brasilitherium and Brasilodon have been discovered. The Triassic continental interior climate was hot and dry, so that typical deposits are red bed sandstones and evaporites. There is no evidence of glaciation near either pole. Pangaea's large size limited the moderating effect of the global ocean; the strong contrast between the Pangea supercontinent and the global ocean triggered intense cross-equatorial monsoons. The Triassic may have been a dry period, but evidence exists that it was punctuated by several episodes of increased rainfall in tropical and subtropical latitudes of the Tethys Sea and its surrounding land. Sediments and fossils suggestive of a more humid climate are known from the Anisian to Ladinian of the Tethysian domain, from the Carnian and Rhaetian of a larger area that includes the Boreal domain, the North
Nematomorpha are a phylum of parasitoid animals superficially similar to nematode worms in morphology, hence the name. Most species range in size from 50 to 100 millimetres long, reaching 2 metres in extreme cases, 1 to 3 millimetres in diameter. Horsehair worms can be discovered in damp areas, such as watering troughs, swimming pools, streams and cisterns; the adult worms are free-living, but the larvae are parasitic on arthropods, such as beetles, mantids and crustaceans. About 351 freshwater species are known and a conservative estimate suggests that there may be about 2000 freshwater species worldwide; the name "Gordian" stems from the legendary Gordian knot. This relates to the fact that nematomorpha tie themselves in knots. Nematomorphs possess an external cuticle without cilia. Internally, they have only longitudinal muscle and a non-functional gut, with no excretory, respiratory or circulatory systems; the nervous system consists of a nerve ring near the anterior end of the animal, a ventral nerve cord running along the body.
Reproductively, they have two distinct sexes, with the internal fertilization of eggs that are laid in gelatinous strings. Adults have cylindrical gonads; the larvae have rings of cuticular hooks and terminal stylets that are believed to be used to enter the hosts. Once inside the host, the larvae live inside the haemocoel and absorb nutrients directly through their skin. Development into the adult form takes weeks or months, the larva moults several times as it grows in size; the adults are free-living in freshwater or marine environments, males and females aggregate into tight balls during mating. In Spinochordodes tellinii and Paragordius tricuspidatus, which have grasshoppers and crickets as their hosts, the infection acts on the infected host's brain; this causes the host insect to drown itself, thus returning the nematomorph to water. P. tricuspidatus is remarkably able to survive the predation of their host, being able to wiggle out of the predator that has eaten the host. There are a few cases including dogs and humans.
Several cases involving Parachordodes, Paragordius, or Gordius have been recorded in human hosts in Japan and China. Nematomorphs can be confused with nematodes mermithid worms. Unlike nematomorphs, mermithids do not have a terminal cloaca. Male mermithids have one or two spicules just before the end apart from having a thinner, smoother cuticle, without areoles and a paler brown colour; the phylum is placed along with the Ecdysozoa clade of moulting organisms that include the Arthropoda. Their closest relatives are the nematodes; the two phyla make up the group Nematoida in the clade Cycloneuralia. During the larval stage, the animals show a resemblance to adult kinorhyncha and some species of Loricifera and Priapulida, all members of the group Scalidophora; the earliest Nematomorph could be Maotianshiania, from the Lower Cambrian. Relationships within the phylum are still somewhat unclear; the five marine species of nematomorph are contained in Nectonematoida. Adults are planktonic and the larvae parasitise decapod crustaceans crabs.
They are characterized by a double row of natotory setae along each side of the body and ventral longitudinal epidermal cords, a spacious and fluid-filled blastocoelom and singular gonads. The 320 remaining species are distributed between two families, comprising seven genera, within order Gordioida. Gordioidean adults are free-living in freshwater or semiterrestrial habitats and larvae parasitise insects orthopterans. Unlike nectonematiodeans, gordioideans lack lateral rows of setae, have a single, ventral epidermal cord and their blastocoels are filled with mesenchyme in young animals but become spacious in older individuals. Pechenik, Jan A.. "Four Phyla of Likely Nematode Relatives". Biology of the Invertebrates. Singapore: Mc-Graw Hill Education. Pp. 452–457. ISBN 978-0-07-127041-0. Baker GL, Capinera JL. "Nematodes and nematomorphs as control agents of grasshoppers and locusts". Memoirs of the Entomological Society of Canada. 171: 157–211. Doi:10.4039/entm129171157-1. Hanelt B, Thomas F, Schmidt-Rhaesa A.
Biology of the phylum Nematomorpha. Advances in Parasitology. 59. Pp. 244–305. Doi:10.1016/S0065-308X59004-3. ISBN 9780120317592. PMID 16182867. Poinar GO Jr. "Nematoda and Nematomorpha". In Thorp JH, Covich AP. Ecology and Classification of North American Freshwater Invertebrates. San Diego, CA: Academic Press. Pp. 249–283. Thorne G. "The hairworm, Gordius robustus Leidy, as a parasite of the Mormon cricket, Anabrus simplex Haldeman". Journal of the Washington Academy of Science. 30: 219–231. Hairworm Biodiversity Survey. Capinera, J. L. Horsehair Worms, Gordian Worms, Gordius spp.. University of Florida IFAS. Published 1999, revised 2005. Nematomorph worm – Behavior modification of cricket by nematomorph worm. YouTube. Gordian Worm discussed on RadioNZ Critter of the Week, 6 November 2015 Minor, M. A. and A. W. Robertson.. Nematoda. Soil Bugs – An Illustrated Guide to New Zealand Soil Invertebrates, http://soilbugs.massey.ac.nz
The ParaHoxozoa are a proposed basal Diploblast/Eumetazoa clade as sister of the Ctenophora. It consists of the Triploblasts/Bilateria as well as the Cnidaria; the Parahoxozoa group was defined based on the presence of several gene classes, as well as Hox/ParaHox-ANTP from which the name of this clade originated. It was proposed and contested that a gene of the same class as the Hox/ParaHox, the NK gene and the Cdx Parahox gene, is present in Porifera; the Parahoxozoa as defined without Porifera may be a monophyletic clade, is continued to be used as such. The original bilateria are hypothesized to be a bottom dwelling worm with a single body opening. A through-gut may have developed with the ctenophora however; the through-gut may have developed from the corners of a single opening with lips fusing. E.g. Acoula resemble the planula larvae of some cnidaria, they are vermiformes. Placozoa has been noted to resemble planula. Sometimes, "Planulozoa" is to the exclusion of Placozoa, but not necessarily.
Triploblasty developed before the Cnidara-Bilateria radiation as well
The Permian is a geologic period and system which spans 47 million years from the end of the Carboniferous Period 298.9 million years ago, to the beginning of the Triassic period 251.902 Mya. It is the last period of the Paleozoic era; the concept of the Permian was introduced in 1841 by geologist Sir Roderick Murchison, who named it after the city of Perm. The Permian witnessed the diversification of the early amniotes into the ancestral groups of the mammals, turtles and archosaurs; the world at the time was dominated by two continents known as Pangaea and Siberia, surrounded by a global ocean called Panthalassa. The Carboniferous rainforest collapse left behind vast regions of desert within the continental interior. Amniotes, who could better cope with these drier conditions, rose to dominance in place of their amphibian ancestors; the Permian ended with the Permian–Triassic extinction event, the largest mass extinction in Earth's history, in which nearly 96% of marine species and 70% of terrestrial species died out.
It would take well into the Triassic for life to recover from this catastrophe. Recovery from the Permian–Triassic extinction event was protracted; the term "Permian" was introduced into geology in 1841 by Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Édouard de Verneuil; the region now lies in the Perm Krai of Russia. Official ICS 2017 subdivisions of the Permian System from most recent to most ancient rock layers are: Lopingian epoch Changhsingian Wuchiapingian Others: Waiitian Makabewan Ochoan Guadalupian epoch Capitanian stage Wordian stage Roadian stage Others: Kazanian or Maokovian Braxtonian stage Cisuralian epoch Kungurian stage Artinskian stage Sakmarian stage Asselian stage Others: Telfordian Mangapirian Sea levels in the Permian remained low, near-shore environments were reduced as all major landmasses collected into a single continent—Pangaea; this could have in part caused the widespread extinctions of marine species at the end of the period by reducing shallow coastal areas preferred by many marine organisms.
During the Permian, all the Earth's major landmasses were collected into a single supercontinent known as Pangaea. Pangaea straddled the equator and extended toward the poles, with a corresponding effect on ocean currents in the single great ocean, the Paleo-Tethys Ocean, a large ocean that existed between Asia and Gondwana; the Cimmeria continent rifted away from Gondwana and drifted north to Laurasia, causing the Paleo-Tethys Ocean to shrink. A new ocean was growing on its southern end, the Tethys Ocean, an ocean that would dominate much of the Mesozoic era. Large continental landmass interiors experience climates with extreme variations of heat and cold and monsoon conditions with seasonal rainfall patterns. Deserts seem to have been widespread on Pangaea; such dry conditions favored gymnosperms, plants with seeds enclosed in a protective cover, over plants such as ferns that disperse spores in a wetter environment. The first modern trees appeared in the Permian. Three general areas are noted for their extensive Permian deposits—the Ural Mountains and the southwest of North America, including the Texas red beds.
The Permian Basin in the U. S. states of Texas and New Mexico is so named because it has one of the thickest deposits of Permian rocks in the world. The climate in the Permian was quite varied. At the start of the Permian, the Earth was still in an ice age. Glaciers receded around the mid-Permian period as the climate warmed, drying the continent's interiors. In the late Permian period, the drying continued although the temperature cycled between warm and cool cycles. Permian marine deposits are rich in fossil mollusks and brachiopods. Fossilized shells of two kinds of invertebrates are used to identify Permian strata and correlate them between sites: fusulinids, a kind of shelled amoeba-like protist, one of the foraminiferans, ammonoids, shelled cephalopods that are distant relatives of the modern nautilus. By the close of the Permian, trilobites and a host of other marine groups became extinct. Terrestrial life in the Permian included diverse plants, fungi and various types of tetrapods; the period saw a massive desert covering the interior of Pangaea.
The warm zone spread in the northern hemisphere. The rocks formed at that time were stained red by iron oxides, the result of intense heating by the sun of a surface devoid of vegetation cover. A number of older types of plants and animals became marginal elements; the Permian began with the Carboniferous flora still flourishing. About the middle of the Permian a major transition in vegetation began; the swamp-loving
Tardigrades are a phylum of water-dwelling, eight-legged, segmented micro-animals. They were first described by the German zoologist Johann August Ephraim Goeze in 1773, who gave them the name of "little water bears"; the name Tardigrada was given in 1777 by the Italian biologist Lazzaro Spallanzani. They have been found everywhere: from mountaintops to mud volcanoes. Tardigrades are among the most resilient known animals, with individual species able to survive extreme conditions that would be fatal to nearly all other known life forms, such as exposure to extreme temperatures, extreme pressures, air deprivation, radiation and starvation. Tardigrades have survived exposure to outer space. About 1,150 known species form a part of the superphylum Ecdysozoa; the group includes fossils dating from 530 million years ago, in the Cambrian period. Tardigrades are about 0.5 mm long when they are grown. They are plump, with four pairs of legs, each ending in claws or sucking disks. Tardigrades are prevalent in mosses and lichens and feed on plant cells and small invertebrates.
When collected, they may be viewed under a low-power microscope, making them accessible to students and amateur scientists. Johann August Ephraim Goeze named the tardigrade kleiner Wasserbär, meaning "little water bear" in German; the name Tardigradum means "slow walker" and was given by Lazzaro Spallanzani in 1776. The name "water bear" comes from the way they reminiscent of a bear's gait; the biggest adults may reach a body length of the smallest below 0.1 mm. Newly hatched tardigrades may be smaller than 0.05 mm. Tardigrades are found on lichens and mosses. Other environments are dunes, beaches and marine or freshwater sediments, where they may occur quite frequently. Tardigrades, in the case of Echiniscoides wyethi, may be found on barnacles. Tardigrades can be found by soaking a piece of moss in water. Tardigrades have barrel-shaped bodies with four pairs of stubby legs. Most range from 0.3 to 0.5 mm in length. The body consists of a head, three body segments each with a pair of legs, a caudal segment with a fourth pair of legs.
The legs are without joints. The cuticle is moulted periodically; the first three pairs of legs are directed downward along the sides, are the primary means of locomotion, while the fourth pair is directed backward on the last segment of the trunk and is used for grasping the substrate. Tardigrades lack a large intermediate region of the body axis. In insects, this corresponds to the abdomen; the whole body, except for the last pair of legs, is made up of just the segments that are homologous to the head region in arthropods. All adult tardigrades of the same species have the same quantity of cells; some species have as many as 40,000 cells in each adult. The body cavity consists of a haemocoel, but the only place where a true coelom can be found is around the gonad. No respiratory organs are found, with gas exchange able to occur across the entirety of the body; some tardigrades have three tubular glands associated with the rectum. Nephridia are absent; the tubular mouth is armed with stylets, which are used to pierce the plant cells, algae, or small invertebrates on which the tardigrades feed, releasing the body fluids or cell contents.
The mouth opens into a triradiate, sucking pharynx. The stylets are lost when the animal molts, a new pair is secreted from a pair of glands that lie on either side of the mouth; the pharynx connects to a short esophagus, to an intestine that occupies much of the length of the body, the main site of digestion. The intestine opens, to an anus located at the terminal end of the body; some species only defecate. The brain develops in a bilaterally symmetric pattern; the brain includes multiple lobes consisting of three bilaterally paired clusters of neurons. The brain is attached to a large ganglion below the esophagus, from which a double ventral nerve cord runs the length of the body; the cord possesses one ganglion per segment, each of which produces lateral nerve fibres that run into the limbs. Many species possess a pair of rhabdomeric pigment-cup eyes, numerous sensory bristles are on the head and body. Tardigrades all possess a buccopharyngeal apparatus which, along with the claws, is used to differentiate among species.
Although some species are parthenogenic, both males and females are present, each with a single gonad located above the intestine. Two ducts run from the testes in males. In contrast, females have a single duct opening either just above the anus or directly into the rectum, which thus forms a cloaca. Tardigrades are oviparous, fertilization is external. Mating occurs during the molt with the eggs being laid inside the shed cuticle of the female and covered with sperm. A