33-572: The Trochozoa are a proposed Lophotrochozoa clade that is a sister clade of Bryozoa and Platyzoa . The clade would include animals in five phyla : the Nemertea , the Annelida , the Mollusca , and the two Brachiozoan phyla, Brachiopoda and Phoronida . Both annelids and molluscs have been suggested as the sister group of Brachiozoa. It has also been proposed that nemerteans are actually
66-409: A "looper" caterpillar . In response to a threat, the head and trunk can be rapidly pulled backwards, or the creeping movement can be reversed. Muscular action is important when the animal turns sideways and during copulation, when two individuals twine around each other. Gastrotrich reproduction and reproductive behaviour has been little studied. That of macrodasiyds probably most represents that of
99-418: A hydrostatic skeleton . 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 body. In some species, there are pores in the pharynx opening to
132-407: A clade of annelids. Gnathifera Gastrotricha Platyhelminthes Brachiopoda Phoronida Mollusca Echiura Clitellata Siboglinidae Sipuncula Nemertea Entoprocta Cycliophora Bryozoa/Ectoprocta Lophotrochozoa Lophotrochozoa ( / l ə ˌ f ɒ t r oʊ k oʊ ˈ z oʊ ə / , "crest/wheel animals") is a clade of protostome animals within
165-471: A de-adhesive agent to sever the connection. In the Macrodasyida, there are additional adhesive glands at the anterior end and on the sides of the body. The body wall consists of a cuticle , an epidermis and longitudinal and circular bands of muscle fibres. In some primitive species, each epidermal cell has a single cilium, a feature shared only by the gnathostomulans . The whole ventral surface of
198-548: A few species are terrestrial and live on land in the film of water surrounding grains of soil . Gastrotrichs are divided into two orders , the Macrodasyida which are marine (except for two species), and the Chaetonotida , some of which are marine and some freshwater. Nearly 800 species of gastrotrich have been described. Gastrotrichs have a simple body plan with a head region, with a brain and sensory organs , and
231-430: 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 organisms and detritus that is found on the seabed and the beds of other water bodies . The majority live on and between particles of sediment or on other submerged surfaces, but
264-402: A single outlet cell which passes the excreted material into the protonephridial duct. As is typical for such small animals, there are no respiratory or circulatory organs. The nervous system is relatively simple. The brain consists of two ganglia , one on either side of the pharynx, connected by a commissure . From these lead a pair of nerve cords which run along either side of the body beside
297-616: A sister group with Gnathifera at the base of Spiralia . Ecdysozoa [REDACTED] Gnathifera [REDACTED] Mollusca [REDACTED] Entoprocta [REDACTED] Gastrotricha [REDACTED] Ectoprocta [REDACTED] Phoronida [REDACTED] Brachiopoda [REDACTED] Annelida [REDACTED] Platyhelminthes [REDACTED] Nemertea [REDACTED] A number of fossil taxa can be identified as early Lophotrochozoans, even if their precise affinity remains contested. However, relevant Cambrian fossils are debated. The clade Lophotrochozoa
330-472: A trunk with a simple gut and the reproductive organs . They have adhesive glands with which they can anchor themselves to the substrate and cilia with which they move around. They feed on detritus, sucking up organic particles with their muscular pharynx . They are hermaphrodites , the marine species producing eggs which develop directly into miniature adults . The freshwater species are parthenogenetic , producing unfertilised eggs, and at least one species
363-460: Is viviparous . Gastrotrichs mature with great rapidity and have lifespans of only a few days. The name gastrotrich comes from Greek γαστήρ, gaster 'stomach' and θρίξ, thrix 'hair'. The name was coined by the Russian zoologist Élie Metchnikoff in 1865. The common name hairyback apparently arises from a mistranslation of gastrotrich . The relationship of gastrotrichs to other phyla
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#1732790834973396-496: Is a cladistic definition (a node-based name), so the affiliation to Lophotrochozoa of spiralian groups not mentioned directly in the definition depends on the topology of the spiralian tree of life, and in some phylogenetic hypotheses, Lophotrochozoa may even be synonymous to Spiralia. Nemertea and Orthonectida (if not directly considered as part of Annelida ) are probably lophotrochozoan phyla; Dicyemida , Gastrotricha , and Platyhelminthes may be lophotrochozoans or placed in
429-442: Is internal. The fertilised eggs are released by rupture of the body wall which afterwards repairs itself. As is the case in most protostomes , development of the embryo is determinate , with each cell destined to become a specific part of the animal's body. At least one species of gastrotrich, Urodasys viviparus , is viviparous . Many species of chaetotonid gastrotrichs reproduce entirely by parthenogenesis . In these species,
462-541: Is named after the two distinct characteristics of its members; the lophophore , a feeding structure consisting of a ciliated crown of tentacles surrounding a mouth, and the developmental stage of the trochophore larva. Lophophorata such as Brachiozoa and Bryozoa have lophophores, while members of Trochozoa such as molluscs and annelids have trochophore larvae, although some may have none. Gastrotricha The gastrotrichs ( phylum Gastrotricha ), commonly referred to as hairybellies or hairybacks , are
495-405: Is not clearly defined as a head but contains the sense organs, brain and pharynx. Cilia are found around the mouth and on the ventral surface of the head and body. The trunk contains the gut and the reproductive organs. At the posterior end of the body are two projections with cement glands that serve in adhesion. This is a double-gland system where one gland secretes the glue and another secretes
528-801: 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 been described. The phylum contains a single class, divided into two orders: the Macrodasyida and the Chaetonotida . Edward Ruppert et al. report that
561-526: The Rouphozoa clade outside Lophotrochozoa; Chaetognatha , Gnathostomulida , Micrognathozoa , and Syndermata are probably gnathiferans and so placed as a basal spiralian clade outside Lophotrochozoa; Cycliophora could be a gnathiferan or a lophotrochozoan phylum. One of the candidate hypotheses is presented below. The Lophotrochozoa has basal Cycliophora and Mollusca groups, and more derived Lophophorate , Nemertea and Annelida groups. With
594-495: The Spiralia . The taxon was established as a monophyletic group based on molecular evidence. The clade includes animals like annelids , molluscs , bryozoans , and brachiopods . Lophotrochozoa was defined in 1995 as the "last common ancestor of the three traditional lophophorate taxa ( brachiopods , bryozoans , and phoronid worms), the mollusks and the annelids , and all of the descendants of that common ancestor". It
627-477: The Macrodasyida are wholly marine, but two rare and poorly known species, Marinellina flagellata and Redudasys fornerise , are known from fresh water. The Chaetonotida comprises both marine and freshwater species. Gastrotrichs vary in size from about 0.06 to 3 mm (0.002 to 0.118 in) in body length. They are bilaterally symmetrical , with a transparent strap-shaped or bowling pin -shaped body, arched dorsally and flattened ventrally. The anterior end
660-408: The adult. The young typically reach sexual maturity in about three days. In the laboratory, Lepidodermella squamatum has lived for up to forty days, producing four or five eggs during the first ten days of life. 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
693-416: 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 anterior portion of which contains sperm -producing cells and the posterior portion producing ova . The sperm is sometimes packaged in spermatophores and is released through male gonopores that open, often temporarily, on
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#1732790834973726-412: The animal may be ciliated or the cilia may be arranged in rows, patches or transverse bands. The cuticle is locally thickened in some gastrotrichs and forms scales, hooks and spines. There is no coelom (body cavity) and the interior of the animal is filled with poorly differentiated connective tissue . In the macrodasyidans, Y-shaped cells, each containing a vacuole , surround the gut and may function as
759-428: The body. Nitrogenous waste is probably excreted through the body wall, as part of respiration, and the protonephridia are believed to function mainly in osmoregulation . Unusually, the protonephridia do not take the form of flame cells , but, instead, the excretory cells consist of a skirt surrounding a series of cytoplasmic rods that in turn enclose a central flagellum . These cells, termed cyrtocytes , connect to
792-653: The clade that consist of the phoronids and the brachiopods. Ecdysozoa [REDACTED] Gnathifera [REDACTED] Gastrotricha [REDACTED] Platyhelminthes [REDACTED] Mesozoa [REDACTED] Cycliophora [REDACTED] Echiura [REDACTED] Clitellata [REDACTED] Siboglinidae [REDACTED] Sipuncula [REDACTED] Mollusca [REDACTED] Brachiopoda [REDACTED] Phoronida [REDACTED] Entoprocta [REDACTED] Ectoprocta [REDACTED] Nemertea [REDACTED] Another study recovers Lophotrochozoa as equivalent to Platytrochozoa, forming
825-462: The introduction of Platytrochozoa and Rouphozoa , one candidate phylogeny is pictured below – though other studies recover a range of alternative possibilities: In the most recent research, the three phyla Cycliophora, Entoprocta and Bryozoa makes up a single clade and are the first to branch off from the other lophotrochozoans. The second split is the molluscs, and the third consists of two sister phyla, annelids and nemerteans. Lastly remains
858-424: The longitudinal muscle bands. The primary sensory organs are the bristles and ciliated tufts of the body surface which function as mechanoreceptors . There are also ciliated pits on the head, simple ciliary photoreceptors and fleshy appendages which act as chemoreceptors . Gastrotrichs are cosmopolitan in distribution. They inhabit the interstitial spaces between particles in marine and freshwater environments,
891-588: The male portions of the reproductive system are degenerate and non-functional, or, in many cases, entirely absent. Though the eggs have a diameter of less than 50 μm , they are still very large in comparison with the animals' size. Some species are capable of laying eggs that remain dormant during times of desiccation or low temperatures; these species, however, are also able to produce regular eggs, which hatch in one to four days, when environmental conditions are more favourable. The eggs of all gastrotrichs undergo direct development and hatch into miniature versions of
924-411: The muscular action of the pharynx. They are themselves eaten by turbellarians and other small macrofauna . Like many microscopic animals, gastrotrich locomotion is primarily powered by hydrostatics , but movement occurs through different methods in different members of the group. Chaetonotids only have adhesive glands at the back and, in them, locomotion typically proceeds in a smooth gliding manner;
957-498: 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 mud, where they thrive even in the presence of hydrogen sulfide . When pools dry up they can survive periods of desiccation as eggs, and some species are capable of forming cysts in harsh conditions. In marine sediments they have been known to reach 364 individuals per 10 cm (1.6 sq in) making them
990-527: The third most common invertebrate in the sediment after nematodes and harpacticoid copepods . In freshwater they may reach a density of 158 individuals per 10 cm (1.6 sq in) and are the fifth most abundant group of invertebrates in the sediment. In marine and freshwater environments, gastrotrichs form part of the benthic community . They are detritivores and are microphagous: they feed by sucking small dead or living organic materials, diatoms , bacteria and small protozoa into their mouths by
1023-477: The underside of the animal, roughly two-thirds of the way along the body. A copulatory organ on the tail collects the sperm and transfers it to the partner's seminal receptacle through the female gonopore. Details of the process and the behaviour involved vary with the species, and there is a range of different accessory reproductive organs. During copulation, the "male" individual uses his copulatory organ to transfer sperm to his partner's gonopore and fertilisation
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1056-400: The ventral surface; these contain valves and may allow egestion of any excess water swallowed while feeding. In the chaetonotidans, the excretory system consists of a single pair of protonephridia , which open through separate pores on the lateral underside of the animal, usually in the midsection of the body. In the macrodasyidans, there are several pairs of these opening along the side of
1089-420: The whole body is propelled forward by the rhythmic action of the cilia on the ventral surface. In the pelagic chaetonotid genus Stylochaeta , however, movement proceeds in jerks as the long, muscle-activated spines are forced rhythmically towards the side of the body. By contrast, with chaetonotids, macrodasyidans typically have multiple adhesive glands and move forward with a creeping action similar to that of
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