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In 2011, a Europa mission was recommended by the U.S. Planetary Science Decadal Survey. In response, NASA commissioned concept studies of a Europa lander in 2011, along with concepts for a Europa flyby (Europa Clipper), and a Europa orbiter. The orbiter element option concentrates on the "ocean" science, while the mult... | Europa (moon) | Wikipedia | 430 | 43127 | https://en.wikipedia.org/wiki/Europa%20%28moon%29 | Physical sciences | Solar System | null |
In the early 2000s, Jupiter Europa Orbiter led by NASA and the Jupiter Ganymede Orbiter led by the ESA were proposed together as an Outer Planet Flagship Mission to Jupiter's icy moons called Europa Jupiter System Mission, with a planned launch in 2020. In 2009 it was given priority over Titan Saturn System Mission. At... | Europa (moon) | Wikipedia | 468 | 43127 | https://en.wikipedia.org/wiki/Europa%20%28moon%29 | Physical sciences | Solar System | null |
So far, there is no evidence that life exists on Europa, but the moon has emerged as one of the most likely locations in the Solar System for potential habitability. Life could exist in its under-ice ocean, perhaps in an environment similar to Earth's deep-ocean hydrothermal vents. Even if Europa lacks volcanic hydroth... | Europa (moon) | Wikipedia | 440 | 43127 | https://en.wikipedia.org/wiki/Europa%20%28moon%29 | Physical sciences | Solar System | null |
Evidence suggests the existence of lakes of liquid water entirely encased in Europa's icy outer shell and distinct from a liquid ocean thought to exist farther down beneath the ice shell, as well as pockets of water that form M-shaped ice ridges when the water freezes on the surface – as in Greenland. If confirmed, the... | Europa (moon) | Wikipedia | 239 | 43127 | https://en.wikipedia.org/wiki/Europa%20%28moon%29 | Physical sciences | Solar System | null |
The Sipuncula or Sipunculida (common names sipunculid worms or peanut worms) is a class containing about 162 species of unsegmented marine annelid worms. Sipuncula was once considered a phylum, but was demoted to a class of Annelida, based on recent molecular work.
Sipunculans vary in size but most species are under ... | Sipuncula | Wikipedia | 407 | 43136 | https://en.wikipedia.org/wiki/Sipuncula | Biology and health sciences | Lophotrochozoa | Animals |
The phylogenetic placement of this group in the past has proved troublesome. Originally classified as annelids, despite the complete lack of segmentation, bristles and other annelid characters, the phylum Sipuncula was later allied with the Mollusca, mostly on the basis of developmental and larval characters. These phy... | Sipuncula | Wikipedia | 470 | 43136 | https://en.wikipedia.org/wiki/Sipuncula | Biology and health sciences | Lophotrochozoa | Animals |
The mouth is located at the anterior end of the animal; in the subclass Sipunculidea, the mouth is surrounded by a mass of 18 to 24 ciliated tentacles, while in the subclass Phascolosomatidea, the tentacles are arranged in an arc above the mouth, surrounding the nuchal organ, also located at the tip of the introvert. T... | Sipuncula | Wikipedia | 275 | 43136 | https://en.wikipedia.org/wiki/Sipuncula | Biology and health sciences | Lophotrochozoa | Animals |
Three genera (Aspidosiphon, Lithacrosiphon and Cloeosiphon) in the Aspidosiphonidae family possess epidermal structures, known as anal and caudal shields. These are patches of thickened, hard plates, and are used for boring into rock; the anal shield is near the anteriorly-located anus on the trunk, just below the intr... | Sipuncula | Wikipedia | 420 | 43136 | https://en.wikipedia.org/wiki/Sipuncula | Biology and health sciences | Lophotrochozoa | Animals |
Circulation
Sipunculans do not have a vascular blood system. Fluid transport and gas exchange are instead accomplished by the coelom, which contains the respiratory pigment haemerythrin, and the separate tentacular system, the two being separated by an elaborate septum. The coelomic fluid contains five types of coelomi... | Sipuncula | Wikipedia | 450 | 43136 | https://en.wikipedia.org/wiki/Sipuncula | Biology and health sciences | Lophotrochozoa | Animals |
In some species, there are simple light-sensitive ocelli associated with the brain. Two organs, likely functioning as a unit for chemoreception are located near its anterior margin; the non-ciliated cerebral organ, which possesses bipolar sensory cells, and the nuchal organ, located posterior to the brain. Similar ligh... | Sipuncula | Wikipedia | 470 | 43136 | https://en.wikipedia.org/wiki/Sipuncula | Biology and health sciences | Lophotrochozoa | Animals |
Most sipunculan species are dioecious. Their gametes are produced in the coelomic lining, where they are released into the coelom to mature. These gametes are then picked up by the metanephridia system and released into the aquatic environment, where fertilisation takes place. In at least one species, Themiste pyroides... | Sipuncula | Wikipedia | 470 | 43136 | https://en.wikipedia.org/wiki/Sipuncula | Biology and health sciences | Lophotrochozoa | Animals |
An unnamed sipunculid worm from the Cambrian period has been discovered in the Burgess Shale in Alberta, Canada, and Lecthaylus has been identified from the Granton Shrimp Bed, near Edinburgh, Scotland, dating to the Silurian period. Trace fossils of burrows that may have been formed by sipunculans have been found from... | Sipuncula | Wikipedia | 320 | 43136 | https://en.wikipedia.org/wiki/Sipuncula | Biology and health sciences | Lophotrochozoa | Animals |
Placozoa ( ; ) is a phylum of free-living (non-parasitic) marine invertebrates. They are blob-like animals composed of aggregations of cells. Moving in water by ciliary motion, eating food by engulfment, reproducing by fission or budding, placozoans are described as "the simplest animals on Earth." Structural and molec... | Placozoa | Wikipedia | 445 | 43139 | https://en.wikipedia.org/wiki/Placozoa | Biology and health sciences | Other | Animals |
In 1893, Italian zoologist Francesco Saverio Monticelli described another animal which he named Treptoplax, the specimens of which he collected from Naples. He gave the species name T. reptans in 1896. Monticelli did not preserve them and no other specimens were found again, as a result of which the identification is r... | Placozoa | Wikipedia | 451 | 43139 | https://en.wikipedia.org/wiki/Placozoa | Biology and health sciences | Other | Animals |
Placozoans have only three anatomical parts as tissue layers inside its body: the upper, intermediate (middle) and lower epithelia. There are at least six different cell types. The upper epithelium is the thinnest portion and essentially comprises flat cells with their cell body hanging underneath the surface, and each... | Placozoa | Wikipedia | 497 | 43139 | https://en.wikipedia.org/wiki/Placozoa | Biology and health sciences | Other | Animals |
Some Trichoplax species contain Rickettsiales bacteria as endosymbionts.
One of the at least 20 described species turned out to have two bacterial endosymbionts; Grellia which lives in the animal's endoplasmic reticulum and is assumed to play a role in the protein and membrane production. The other endosymbiont is the ... | Placozoa | Wikipedia | 410 | 43139 | https://en.wikipedia.org/wiki/Placozoa | Biology and health sciences | Other | Animals |
Traditionally, classification was based on their level of organization, i.e., they possess no tissues or organs. However this may be as a result of secondary loss and thus is inadequate to exclude them from relationships with more complex animals. More recent work has attempted to classify them based on the DNA sequenc... | Placozoa | Wikipedia | 391 | 43139 | https://en.wikipedia.org/wiki/Placozoa | Biology and health sciences | Other | Animals |
Infolding of the epithelium led to the formation of an internal system of ducts and thus to the development of a modified gallertoid from which the sponges (Porifera), Cnidaria and Ctenophora subsequently developed.
Other gallertoids, according to this model, made the transition over time to a benthic mode of life; th... | Placozoa | Wikipedia | 244 | 43139 | https://en.wikipedia.org/wiki/Placozoa | Biology and health sciences | Other | Animals |
Three different life strategies have accordingly led to three different possible lines of development:
Animals that live interstitially in the sand of the ocean floor were responsible for the fossil crawling traces that are considered the earliest evidence of animals; and are detectable even prior to the dawn of the E... | Placozoa | Wikipedia | 401 | 43139 | https://en.wikipedia.org/wiki/Placozoa | Biology and health sciences | Other | Animals |
Should any of the analyses presented above turn out to be correct, Trichoplax adhaerens would be the oldest branch of the multicellular animals, and a relic of the Ediacaran fauna, or even the pre-Ediacara fauna. Although very successful in their ecological niche, due to the absence of extracellular matrix and basal la... | Placozoa | Wikipedia | 501 | 43139 | https://en.wikipedia.org/wiki/Placozoa | Biology and health sciences | Other | Animals |
One possible scenario inspired by the proposed hypothesis starts with the idea that the monociliated cells of the epitheloid in Trichoplax adhaerens evolved by reduction of the collars in the collar cells (choanocytes) of sponges as the hypothesized ancestors of the Placozoa abandoned a filtering mode of life. The epit... | Placozoa | Wikipedia | 494 | 43139 | https://en.wikipedia.org/wiki/Placozoa | Biology and health sciences | Other | Animals |
In this cladogram the Epitheliozoa and Eumetazoa are synonyms to each other and to the Diploblasts, and the Ctenophora are basal to them.
An argument raised against the proposed scenario is that it leaves morphological features of the animals completely out of consideration. The extreme degree of simplification that w... | Placozoa | Wikipedia | 287 | 43139 | https://en.wikipedia.org/wiki/Placozoa | Biology and health sciences | Other | Animals |
Symbion is a genus of commensal aquatic animals, less than 0.5 mm wide, found living attached to the mouthparts of cold-water lobsters. They have sac-like bodies, and three distinctly different forms in different parts of their two-stage life-cycle. They appear so different from other animals that they were assigned th... | Symbion | Wikipedia | 508 | 43140 | https://en.wikipedia.org/wiki/Symbion | Biology and health sciences | Spiralia | Animals |
Reproduction
Symbion reproduces both asexually and sexually, and has a complex reproduction cycle, a strategy evolved to produce as many offspring as possible that can survive and find a new host when the lobster they live on sheds its shell. The asexual individuals are the largest ones. The sexual individuals do not ... | Symbion | Wikipedia | 341 | 43140 | https://en.wikipedia.org/wiki/Symbion | Biology and health sciences | Spiralia | Animals |
By late winter, when the large feeding individuals in the colony have males attached to their bodies, they start making females. Each female has a single egg inside her. When she is about to be born, one of the two dwarf males fertilizes her when she comes out. The fertilized female finds herself a place on the host's ... | Symbion | Wikipedia | 201 | 43140 | https://en.wikipedia.org/wiki/Symbion | Biology and health sciences | Spiralia | Animals |
An echinoderm () is any animal of the phylum Echinodermata (), which includes starfish, brittle stars, sea urchins, sand dollars and sea cucumbers, as well as the sessile sea lilies or "stone lilies". While bilaterally symmetrical as larvae, as adults echinoderms are recognisable by their usually five-pointed radial sy... | Echinoderm | Wikipedia | 478 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
While Echinodermata has been in common use since the mid-1800s, several other names had been proposed. Notably, F. A. Bather called the phylum "Echinoderma" (apparently after Latreille, 1825) in his 1900 treatise on the phylum, but this name now refers to a fungus.
Diversity
There are about 7,600 extant species of e... | Echinoderm | Wikipedia | 504 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
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 o... | Echinoderm | Wikipedia | 460 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
One characteristic of most echinoderms 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 ... | Echinoderm | Wikipedia | 362 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
The organisation of the water vascular system is somewhat different in ophiuroids, where 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 ... | Echinoderm | Wikipedia | 489 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
The coelomic cavities of echinoderms are complex. Aside from the water vascular system, echinoderms have a haemal coelom, a perivisceral 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, hydroc... | Echinoderm | Wikipedia | 323 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
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 us... | Echinoderm | Wikipedia | 501 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
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 the course of several months. Sea urchins constantly replace spines los... | Echinoderm | Wikipedia | 425 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
Some echinoderms brood their eggs. This 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 p... | Echinoderm | Wikipedia | 462 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
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... | Echinoderm | Wikipedia | 361 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
The larvae pass through several stages, which have specific names derived from the taxonomic names of the adults or from their appearance. For example, 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 'brac... | Echinoderm | Wikipedia | 456 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
Echinoderms primarily use their tube feet to move about, though some sea urchins also use their spines. The tube feet typically have 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 fe... | Echinoderm | Wikipedia | 496 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
The majority of feather stars (also called Comatulida or "unstalked crinoids") and some stalked forms are motile. Several stalked crinoid species are sessile, attached permanently to the substratum. Movement in most sea lilies is limited to bending (their stems can bend) and rolling and unrolling their arms; a few spec... | Echinoderm | Wikipedia | 449 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
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 dolla... | Echinoderm | Wikipedia | 473 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
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; the great quantity of eggs... | Echinoderm | Wikipedia | 500 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
Echinoderms sometimes have large population swings which can transform ecosystems. In 1983, for example, the mass mortality of the tropical sea urchin Diadema antillarum in the Caribbean caused a change from a coral-dominated reef system to an alga-dominated one. Sea urchins are among the main herbivores on reefs and t... | Echinoderm | Wikipedia | 438 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
Echinoderms are the sister group of the Hemichordata, with which they form the crown group Ambulacraria. Two taxa of uncertain placement, Vetulocystida and Yanjiahella, have each been proposed as either stem-group echinoderms or stem-group ambulacrarians. Vetulocystids have also been proposed as stem-group chordates,... | Echinoderm | Wikipedia | 475 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
Carpoidea for Homalozoa, giving rise to the term "carpoids"
Cincta as either the senior synonym of or sole order within Homostelea
Soluta as either the senior synonym of or sole order within Homoiostelea
Calcichordata , a subphylum effectively identical to Stylophora that was central to the now-disproven calcicho... | Echinoderm | Wikipedia | 381 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
Those who find pentaradiality to be derived incorporate the recently-discovered fossils Ctenoimbricata (seen as a possible sister to all other echinoderms) and Helicocystis (seen as bridging the triradial helicoplacoids and the pentaradial crown group). They cite research indicating that the early appearance of pentara... | Echinoderm | Wikipedia | 405 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
The oldest potential echinoderm fossil is Arkarua from the late Ediacaran of Australia circa 555 Ma. These fossils are disc-like, with radial ridges on the rim and a five-pointed central depression marked with radial lines. However, the fossils have no stereom or internal structure indicating a water vascular system, s... | Echinoderm | Wikipedia | 472 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
The first known echinoderms were non-motile, but evolved into animals able to move freely. These soon developed endoskeletal plates with stereom structure, and external ciliary grooves for feeding. The Paleozoic echinoderms were globular, attached to the substrate and were orientated with their oral surfaces facing upw... | Echinoderm | Wikipedia | 444 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
Because of their robust larval growth, sea urchins are widely used in research, particularly as model organisms in developmental biology and ecotoxicology. Strongylocentrotus purpuratus and Arbacia punctulata are used for this purpose in embryological studies. The large size and the transparency of the eggs enables the... | Echinoderm | Wikipedia | 374 | 43143 | https://en.wikipedia.org/wiki/Echinoderm | Biology and health sciences | Echinoderms | null |
The gastrotrichs (phylum Gastrotricha), commonly referred to as hairybellies or hairybacks, are a group of microscopic (0.06–3.0 mm), cylindrical, acoelomate animals, and are widely distributed and abundant in freshwater and marine environments. They are mostly benthic and live within the periphyton, the layer of tiny ... | Gastrotrich | Wikipedia | 402 | 43145 | https://en.wikipedia.org/wiki/Gastrotrich | Biology and health sciences | Platyzoa | Animals |
The relationship of gastrotrichs to other phyla is unclear. Morphology suggests that they are close to the Gnathostomulida, the Rotifera, or the Nematoda. On the other hand, genetic studies place them as close relatives of the Platyhelminthes, the Ecdysozoa or the Lophotrochozoa. As of 2011, around 790 species have be... | Gastrotrich | Wikipedia | 495 | 43145 | https://en.wikipedia.org/wiki/Gastrotrich | Biology and health sciences | Platyzoa | Animals |
The mouth is at the anterior end and opens into an elongated muscular pharynx with a triangular or Y-shaped lumen, lined by myoepithelial cells. The pharynx opens into a cylindrical intestine, which is lined with glandular and digestive cells. The anus is located on the ventral surface close to the posterior of the bod... | Gastrotrich | Wikipedia | 433 | 43145 | https://en.wikipedia.org/wiki/Gastrotrich | Biology and health sciences | Platyzoa | Animals |
Gastrotrichs are cosmopolitan in distribution. They inhabit the interstitial spaces between particles in marine and freshwater environments, the surfaces of aquatic plants and other submerged objects and the surface film of water surrounding soil particles on land. They are also found in stagnant pools and anaerobic mu... | Gastrotrich | Wikipedia | 457 | 43145 | https://en.wikipedia.org/wiki/Gastrotrich | Biology and health sciences | Platyzoa | Animals |
Gastrotrich reproduction and reproductive behaviour has been little studied. That of macrodasiyds probably most represents that of the ancestral lineage and these more primitive gastrotrichs are simultaneous hermaphrodites, possessing both male and female sex organs. There is generally a single pair of gonads, the ante... | Gastrotrich | Wikipedia | 478 | 43145 | https://en.wikipedia.org/wiki/Gastrotrich | Biology and health sciences | Platyzoa | Animals |
Gastrotrichs demonstrate eutely, each species having an invariant genetically fixed number of cells as adults. Cell division ceases at the end of embryonic development and further growth is solely due to cell enlargement.
Classification
Gastrotricha is divided into two orders and a number of families: | Gastrotrich | Wikipedia | 61 | 43145 | https://en.wikipedia.org/wiki/Gastrotrich | Biology and health sciences | Platyzoa | Animals |
Order Macrodasyida Remane, 1925 [Rao and Clausen, 1970]
Family Cephalodasyidae Hummon & Todaro, 2010
Genus Cephalodasys Remane, 1926
Genus Dolichodasys Gagne, 1977
Genus Megadasys Schmidt, 1974
Genus Mesodasys Remane, 1951
Genus Paradasys Remane, 1934
Genus Pleurodasys Remane, 1927
Family Dactylopodolidae Strand, 1929
... | Gastrotrich | Wikipedia | 499 | 43145 | https://en.wikipedia.org/wiki/Gastrotrich | Biology and health sciences | Platyzoa | Animals |
Genus Chordodasiopsis Todaro, Guidi, Leasi & Tongiorgi, 2006
Genus Xenodasys Swedmark, 1967 | Gastrotrich | Wikipedia | 29 | 43145 | https://en.wikipedia.org/wiki/Gastrotrich | Biology and health sciences | Platyzoa | Animals |
Incertae sedis
Genus Marinellina Ruttner-Kolisko, 1955
Order Chaetonotida Remane, 1925 [Rao and Clausen, 1970]
Suborder Multitubulatina d'Hondt, 1971
Family Neodasyidae Remane, 1929
Genus Neodasys Remane, 1927
Suborder Paucitubulatina d'Hondt, 1971
Family Chaetonotidae Gosse, 1864
Subfamily Chaetonotinae Kisielewski... | Gastrotrich | Wikipedia | 506 | 43145 | https://en.wikipedia.org/wiki/Gastrotrich | Biology and health sciences | Platyzoa | Animals |
Hemichordata ( ) 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 ... | Hemichordate | Wikipedia | 482 | 43146 | https://en.wikipedia.org/wiki/Hemichordate | Biology and health sciences | Other | Animals |
The prosome of pterobranchs is specialized into 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, con... | Hemichordate | Wikipedia | 479 | 43146 | https://en.wikipedia.org/wiki/Hemichordate | Biology and health sciences | Other | Animals |
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 a larval stage that feeds on plankton before turning into an adult worm. The Pterobranc... | Hemichordate | Wikipedia | 171 | 43146 | https://en.wikipedia.org/wiki/Hemichordate | Biology and health sciences | Other | Animals |
Ptychodera flava
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 is equal and equatorial so th... | Hemichordate | Wikipedia | 372 | 43146 | https://en.wikipedia.org/wiki/Hemichordate | Biology and health sciences | Other | Animals |
Saccoglossus kowalevskii
Eggs of S. kowalevskii are oval in shape and become spherical in shape after fertilization. The first cleavage occurs from 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 an... | Hemichordate | Wikipedia | 303 | 43146 | https://en.wikipedia.org/wiki/Hemichordate | Biology and health sciences | Other | Animals |
Genetic control of dorsal-ventral hemichordate patterning
Much of the genetic work done on hemichordates has been done to make comparison with chordates, so many of 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... | Hemichordate | Wikipedia | 512 | 43146 | https://en.wikipedia.org/wiki/Hemichordate | Biology and health sciences | Other | Animals |
Phylogeny
A phylogenetic tree showing the position of the hemichordates is:
The internal relationships within the hemichordates are shown below. The tree is based on 16S +18S rRNA sequence data and phylogenomic studies from multiple sources. | Hemichordate | Wikipedia | 55 | 43146 | https://en.wikipedia.org/wiki/Hemichordate | Biology and health sciences | Other | Animals |
Acanthocephala (Greek , 'thorn' + , 'head') is a group of parasitic worms known as acanthocephalans, thorny-headed worms, or spiny-headed worms, characterized by the presence of an eversible proboscis, armed with spines, which it uses to pierce and hold the gut wall of its host. Acanthocephalans have complex life cy... | Acanthocephala | Wikipedia | 500 | 43147 | https://en.wikipedia.org/wiki/Acanthocephala | Biology and health sciences | Platyzoa | Animals |
A study of the gene order in the mitochondria suggests that Seisonidea and Acanthocephala are sister clades and that the Bdelloidea are the sister clade to this group.
Currently the phylum is divided into four classes – Palaeacanthocephala, Archiacanthocephala, Polyacanthocephala and Eoacanthocephala. The monophyletic... | Acanthocephala | Wikipedia | 419 | 43147 | https://en.wikipedia.org/wiki/Acanthocephala | Biology and health sciences | Platyzoa | Animals |
Like the body, the proboscis is hollow, and its cavity is separated from the body cavity by a septum or proboscis sheath. Traversing the cavity of the proboscis are muscle-strands inserted into the tip of the proboscis at one end and into the septum at the other. Their contraction causes the proboscis to be invaginate... | Acanthocephala | Wikipedia | 382 | 43147 | https://en.wikipedia.org/wiki/Acanthocephala | Biology and health sciences | Platyzoa | Animals |
Except for the absence of the longitudinal fibres the skin of the proboscis resembles that of the body, but the fluid-containing tubules of the proboscis are shut off from those of the body. The canals of the proboscis open into a circular vessel which runs round its base. From the circular canal two sac-like projectio... | Acanthocephala | Wikipedia | 435 | 43147 | https://en.wikipedia.org/wiki/Acanthocephala | Biology and health sciences | Platyzoa | Animals |
In the female, the ovaries are found, like the testes, as rounded bodies along the ligament. From the ovaries, masses of ova dehisce into the body cavity, floating in its fluids for fertilization by male's sperm. After fertilization, each egg contains a developing embryo. (These embryos hatch into first stage larva.) T... | Acanthocephala | Wikipedia | 497 | 43147 | https://en.wikipedia.org/wiki/Acanthocephala | Biology and health sciences | Platyzoa | Animals |
Thorny-headed worms begin their life cycle inside invertebrates that reside in marine or freshwater systems. One example is Polymorphus paradoxus. Gammarus lacustris, a small crustacean that inhabits ponds and rivers, is one invertebrate that P. paradoxus may occupy; ducks are one of the definitive hosts.
This crus... | Acanthocephala | Wikipedia | 476 | 43147 | https://en.wikipedia.org/wiki/Acanthocephala | Biology and health sciences | Platyzoa | Animals |
Economic impact
Acanthocephalosis, a disease caused by Acanthacephalus infection, is prevalent in aquaculture, occurring in Atlantic salmon, rainbow and brown trout, tilapia, and tambaqui. Increasing occurrence in Brazilian farming of tambaqui has been reported, and in 2003 Acanthacephalus was first reported in cultur... | Acanthocephala | Wikipedia | 404 | 43147 | https://en.wikipedia.org/wiki/Acanthocephala | Biology and health sciences | Platyzoa | Animals |
Loricifera (from Latin, lorica, corselet (armour) + ferre, to bear) is a phylum of very small to microscopic marine cycloneuralian sediment-dwelling animals with 43 described species. and approximately 100 more that have been collected and not yet described. Their sizes range from 100 μm to .
They are characterised by... | Loricifera | Wikipedia | 448 | 43148 | https://en.wikipedia.org/wiki/Loricifera | Biology and health sciences | Ecdysozoa | Animals |
Morphological studies have traditionally placed the phylum in the Vinctiplicata with the Priapulida; this plus the Kinorhyncha constitutes the taxon Scalidophora. The three phyla share four characters in common – chitinous cuticle, rings of scalids on the introvert, flosculi, and two rings of introvert retracts. Howeve... | Loricifera | Wikipedia | 352 | 43148 | https://en.wikipedia.org/wiki/Loricifera | Biology and health sciences | Ecdysozoa | Animals |
The newly reported animals complete their life cycle in the total absence of light and oxygen, and they are less than a millimetre in size. They were collected from a deep basin at the bottom of the Mediterranean Sea, where they inhabit a nearly salt-saturated brine that, because of its density (> 1.2 g/cm3), does not ... | Loricifera | Wikipedia | 173 | 43148 | https://en.wikipedia.org/wiki/Loricifera | Biology and health sciences | Ecdysozoa | Animals |
The Chaetognatha or chaetognaths (meaning bristle-jaws) are a phylum of predatory marine worms that are a major component of plankton worldwide. Commonly known as arrow worms, they are mostly nektonic; however about 20% of the known species are benthic, and can attach to algae and rocks. They are found in all marine ... | Chaetognatha | Wikipedia | 385 | 43171 | https://en.wikipedia.org/wiki/Chaetognatha | Biology and health sciences | Spiralia | Animals |
The mouth opens into a muscular pharynx, which contains glands to lubricate the passage of food. From here, a straight intestine runs the length of the trunk to an anus just forward of the tail. The intestine is the primary site of digestion and includes a pair of diverticula near the anterior end. Materials are moved ... | Chaetognatha | Wikipedia | 346 | 43171 | https://en.wikipedia.org/wiki/Chaetognatha | Biology and health sciences | Spiralia | Animals |
Chaetognaths have two compound eyes, each consisting of a number of pigment-cup ocelli fused together; some deep-sea and troglobitic species have unpigmented or absent eyes. In addition, there are a number of sensory bristles arranged in rows along the side of the body, where they probably perform a function similar to... | Chaetognatha | Wikipedia | 504 | 43171 | https://en.wikipedia.org/wiki/Chaetognatha | Biology and health sciences | Spiralia | Animals |
Chaetognaths swim in short bursts using a dorso-ventral undulating body motion, where their tail fin assists with propulsion and the body fins with stabilization and steering. Muscle movements have been described as among the fastest in metazoans. Muscles are directly excitable by electrical currents or strong K+ solut... | Chaetognatha | Wikipedia | 462 | 43171 | https://en.wikipedia.org/wiki/Chaetognatha | Biology and health sciences | Spiralia | Animals |
Behaviour
Little is known of arrow worms' behaviour and physiology, due to the complexity in culturing them and reconstructing their natural habitat. It is known that they feed more frequently with higher temperatures. Planktonic chaetognaths often must swim continuously, with a "hop and sink" behaviour, to keep thems... | Chaetognatha | Wikipedia | 467 | 43171 | https://en.wikipedia.org/wiki/Chaetognatha | Biology and health sciences | Spiralia | Animals |
The highest density of chaetognaths is observed in the photic zone of shallow waters. Larger chaetognath species tend to live deeper in water, but spend their juvenile stages higher in the water column. Arrow worms however engage in diel vertical migration, spending the day at lower depths to avoid predators, and comin... | Chaetognatha | Wikipedia | 396 | 43171 | https://en.wikipedia.org/wiki/Chaetognatha | Biology and health sciences | Spiralia | Animals |
External
The evolutionary relationships of chaetognaths have long been enigmatic. Charles Darwin remarked that arrow worms were "remarkable for the obscurity of their affinities". Chaetognaths in the past have been traditionally, but erroneously, classed as deuterostomes by embryologists due to deuterostome-like featur... | Chaetognatha | Wikipedia | 379 | 43171 | https://en.wikipedia.org/wiki/Chaetognatha | Biology and health sciences | Spiralia | Animals |
Fossil record
Due to their soft bodies, chaetognaths fossilize poorly. Even so, several fossil chaetognath species have been described. Chaetognaths first appear during the Cambrian Period. Complete body fossils have been formally described from the Lower Cambrian Maotianshan shales of Yunnan, China (Eognathacantha er... | Chaetognatha | Wikipedia | 487 | 43171 | https://en.wikipedia.org/wiki/Chaetognatha | Biology and health sciences | Spiralia | Animals |
The term "chaetognath" has been coined in 1856 by Rudolf Leuckart. He was also the first to propose that the genus Sagitta belonged to a separate group: «At the moment, it seems most natural to regard the Sagittas as representatives of a small group of their own that makes the transition from the real annelids (first o... | Chaetognatha | Wikipedia | 269 | 43171 | https://en.wikipedia.org/wiki/Chaetognatha | Biology and health sciences | Spiralia | Animals |
In 2018, reanalysis of electron microscopy photographs from the 1980s allowed scientists to identify a giant virus (Meelsvirus) infecting Adhesisagitta hispida; its site of multiplication is nuclear and the virions (length: 1.25 μm) are enveloped. In 2019, reanalysis of other previous studies has shown that structures ... | Chaetognatha | Wikipedia | 333 | 43171 | https://en.wikipedia.org/wiki/Chaetognatha | Biology and health sciences | Spiralia | Animals |
Conodonts (Greek kōnos, "cone", + odont, "tooth") are an extinct group of jawless vertebrates, classified in the class Conodonta. They are primarily known from their hard, mineralised tooth-like structures called "conodont elements" that in life were present in the oral cavity and used to process food. Rare soft tissu... | Conodont | Wikipedia | 480 | 43175 | https://en.wikipedia.org/wiki/Conodont | Biology and health sciences | Prehistoric agnathae and early chordates | Animals |
Conodonta and Conodontophorida are by far the most common scientific names used to refer to conodonts, though inconsistencies regarding their taxonomic rank still persist. Bengtson (1976)'s research on conodont evolution identified three morphological tiers of early conodont-like fossils: protoconodonts, paraconodonts,... | Conodont | Wikipedia | 421 | 43175 | https://en.wikipedia.org/wiki/Conodont | Biology and health sciences | Prehistoric agnathae and early chordates | Animals |
Conodont elements
For a long time, the function and arrangement of conodont elements was enigmatic, since the whole animal was soft-bodied, with the sole exception of the mineralized elements. Upon the conodont animal's demise, the soft tissues would decompose and the individual conodont elements would separate. Howev... | Conodont | Wikipedia | 433 | 43175 | https://en.wikipedia.org/wiki/Conodont | Biology and health sciences | Prehistoric agnathae and early chordates | Animals |
They are widely used in biostratigraphy. Conodont elements are also used as paleothermometers, a proxy for thermal alteration in the host rock, because under higher temperatures, the phosphate undergoes predictable and permanent color changes, measured with the conodont alteration index. This has made them useful for p... | Conodont | Wikipedia | 430 | 43175 | https://en.wikipedia.org/wiki/Conodont | Biology and health sciences | Prehistoric agnathae and early chordates | Animals |
In P elements, a pectiniform (comb-shaped) row of cusps transitions into a broad flat or ridged platform moving towards the base of the element. Platforms and cusps are only found along one side of the structure. Individual elements oriented vertically and arranged in pairs, with platforms and cusps pointing towards th... | Conodont | Wikipedia | 452 | 43175 | https://en.wikipedia.org/wiki/Conodont | Biology and health sciences | Prehistoric agnathae and early chordates | Animals |
Diet
Because they are associated with the oral region of the conodont animal, it is accepted that conodont elements are used in the acquisition of food. Two primary hypotheses have arisen as to how this is accomplished. One hypothesis proposed that elements acted as support structures for filamentous soft-tissues. The... | Conodont | Wikipedia | 502 | 43175 | https://en.wikipedia.org/wiki/Conodont | Biology and health sciences | Prehistoric agnathae and early chordates | Animals |
Lifestyle
Studies have concluded that conodonts taxa occupied both pelagic (open ocean) and nektobenthic (swimming above the sediment surface) niches. The preserved musculature suggests that some conodonts (Promissum at least) were efficient cruisers, but incapable of bursts of speed. Based on isotopic evidence, some ... | Conodont | Wikipedia | 506 | 43175 | https://en.wikipedia.org/wiki/Conodont | Biology and health sciences | Prehistoric agnathae and early chordates | Animals |
Only a few studies approach the question of conodont ingroup relationships from a cladistic perspective, as informed by phylogenetic analyses. One of the broadest studies of this nature was the analysis of Donoghue et al. (2008), which focused on "complex" conodonts (Prioniodontida and other descendant groups):
Evolut... | Conodont | Wikipedia | 308 | 43175 | https://en.wikipedia.org/wiki/Conodont | Biology and health sciences | Prehistoric agnathae and early chordates | Animals |
Taxonomy
Conodonta taxonomy based on Sweet (1988), Sweet & Donoghue (2001), and Mikko's Phylogeny Archive.
Class Conodonta Pander, 1856 [Conodontophorida Eichenberg, 1930; "euconodonts" Bengtson, 1976]
Cavidonti Sweet, 1988
Order Belodellida? Sweet, 1988
Ansellidae? Fåhraeus & Hunter, 1985
Belodellidae Khodalevich & ... | Conodont | Wikipedia | 499 | 43175 | https://en.wikipedia.org/wiki/Conodont | Biology and health sciences | Prehistoric agnathae and early chordates | Animals |
Multioistodontidae Harris, 1964 [Dischidognathidae]
Oistodontidae Lindström, 1970 [Juanognathidae Bergström, 1981]
Periodontidae Lindström, 1970
Rhipidognathidae Lindström, 1970 sensu Sweet, 1988
Prioniodontidae Bassler, 1925
Phragmodontidae Bergström, 1981 [Cyrtoniodontinae Hass, 1959]
Plectodinidae Sweet, 1988
Pygodo... | Conodont | Wikipedia | 499 | 43175 | https://en.wikipedia.org/wiki/Conodont | Biology and health sciences | Prehistoric agnathae and early chordates | Animals |
Spathognathodontidae Hass, 1959 [Ozarkodinidae Dzik, 1976]
Pterospathodontidae Cooper, 1977 [Carniodontidae]
Kockelellidae Klapper, 1981 [Caenodontontidae]
Polygnathidae Bassler, 1925 [?Eopolygnathidae Bardashev, Weddige & Ziegler, 2002]
Palmatolepidae Sweet, 1988
Hindeodontidae (Hass, 1959)
Elictognathidae Austin & Rh... | Conodont | Wikipedia | 158 | 43175 | https://en.wikipedia.org/wiki/Conodont | Biology and health sciences | Prehistoric agnathae and early chordates | Animals |
Gnathostomulids, or jaw worms, are a small phylum of nearly microscopic marine animals. They inhabit sand and mud beneath shallow coastal waters and can survive in relatively anoxic environments. They were first recognised and described in 1956.
Anatomy
Most gnathostomulids measure in length. They are often slender t... | Gnathostomulid | Wikipedia | 442 | 43177 | https://en.wikipedia.org/wiki/Gnathostomulid | Biology and health sciences | Spiralia | Animals |
Reproduction
Gnathostomulids are simultaneous hermaphrodites. Each individual possesses a single ovary and one or two testes. After fertilization, the single egg ruptures through the body wall and adheres to nearby sand particles; the parent is able to rapidly heal the resulting wound. The egg hatches into a miniature ... | Gnathostomulid | Wikipedia | 230 | 43177 | https://en.wikipedia.org/wiki/Gnathostomulid | Biology and health sciences | Spiralia | Animals |
Lobopodians are members of the informal group Lobopodia (from the Greek, meaning "blunt feet"), or the formally erected phylum Lobopoda Cavalier-Smith (1998). They are panarthropods with stubby legs called lobopods, a term which may also be used as a common name of this group as well. While the definition of lobopodian... | Lobopodia | Wikipedia | 275 | 43184 | https://en.wikipedia.org/wiki/Lobopodia | Biology and health sciences | Ecdysozoa | Animals |
The Lobopodian concept varies from author to author. Its most general sense refers to a suite of mainly Cambrian worm-like panarthropod taxa possessing lobopods – for example, Aysheaia, Hallucigenia, and Xenusion – which were traditionally united as "Xenusians" or "Xenusiids" (class Xenusia). Certain Dinocaridid genera... | Lobopodia | Wikipedia | 268 | 43184 | https://en.wikipedia.org/wiki/Lobopodia | Biology and health sciences | Ecdysozoa | Animals |
An alternative, broader definition of Lobopodia would also incorporate Onychophora and Tardigrada, the two living panarthropod phyla which still bear lobopodous limbs. This definition, corresponding to "C", is a morphological one, depending on the superficial similarity of appendages (the "lobopods"). Thus, it is parap... | Lobopodia | Wikipedia | 409 | 43184 | https://en.wikipedia.org/wiki/Lobopodia | Biology and health sciences | Ecdysozoa | Animals |
The better-known genera include Aysheaia, which was discovered in the Canadian Burgess Shale, and Hallucigenia, known from both the Chenjiang Maotianshan Shale and the Burgess Shale. Aysheaia pedunculata has a morphology apparently basic for lobopodians — for example, a significantly annulated cuticle, a terminal mouth... | Lobopodia | Wikipedia | 483 | 43184 | https://en.wikipedia.org/wiki/Lobopodia | Biology and health sciences | Ecdysozoa | Animals |
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