Misplaced Pages

#201798

18-444: It is a polyphyletic group. Although invertebrate experts do not necessarily agree on these categorizations, groups that are generally incorporated into Aschelminthes include: Gnathifera Nematoidea Scalidophora In addition, Cycliophora , Entoprocta and Tardigrada are sometimes included. Gnathifera (clade) Gnathifera (from the Greek gnáthos , “jaw”, and

36-428: A distinct lineage with no close relatives, were identified as gnathiferans in 2019. A similar grouping, Acanthognatha, was suggested in 1998 to unite gastrotrichs with gnathostomulids and rotifers. However, gastrotrichs are more closely related to lophotrochozoans than gnathiferans. Cycliophora Symbion is a genus of commensal aquatic animals, less than 0.5 mm wide, found living attached to

54-427: A new host when the lobster they live on sheds its shell. The asexual individuals are the largest ones. The sexual individuals do not eat. During the autumn they make copies of themselves, where a new individual grows inside the parent body, one offspring at the time. The new offspring attach themselves to an available spot on the lobster, begin to feed and eventually start making new copies of themselves. In early winter,

72-427: A place on the host's whiskers where she attaches herself. Inside her the developing embryo extracts all the nutrients it needs to grow from its mother, and by the time it is ready to be born, all that remains of the mother is an empty husk. This new offspring is a strong swimmer unlike all the other forms in the colony, and those who succeed in finding a new host will attach themselves to its mouthparts, where it will grow

90-748: Is a member of Spiralia. It is the sister taxon of a clade comprising all other spiralians. An alternative phylogeny place Gnathifera into a main spiralian clade Platyzoa s.l. as sister clade to Mesozoa and Platyhelminthes . Before the cladistic era, most gnathiferans were regarded as aschelminths , a grouping now recognized as polyphyletic . Chaetognaths exhibit numerous morphological similarities to rotifers, suggesting that they may be sister taxa. However, based on molecular data, micrognathozoans may be more closely related to rotifers than chaetognaths. Rotifera comprises four subclades: Seisonida , Acanthocephala , Bdelloidea , and Monogononta . Acanthocephalans were traditionally excluded from Rotifera, but it

108-450: Is an adaptation that allows two mature individuals to fit inside the body of the parent male. Two males increases their chances to fertilize a female. 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

126-423: Is now known that rotifers are paraphyletic without including acanthocephalans. Some taxonomists call the clade of rotifers including acanthocephalans Syndermata, but others continue to use Rotifera and regard acanthocephalans as rotifers. Numerous hypotheses of rotifer interrelationships exist. The enigmatic phylum Cycliophora may belong to Gnathifera, but other studies suggest that it is more closely related to

144-437: Is still not finally settled. Currently it is placed in the clade Polyzoa along with the phyla Ectoprocta and Entoprocta , based on genetic analysis. Symbion pandora has a bilateral , sac-like body with no coelom . There are three basic life stages: 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

162-620: Is transient and only forms during defecation. Unlike other gnathiferans, in chaetognaths and the extinct Amiskwia the anus is located on the ventral surface in a subterminal position. Both Gnathostomulida and Micrognathozoa are acoelomates , rotifers are pseudocoelomates , and chaetognathans have a true coelom . Gnathostomulidans and chaetognathans are hermaphrodites , and Micrognathozoa appears to be parthenogenetic as no males have ever been observed, but it has been suggested undiscovered dwarf males could be responsible for fertilization. In rotifers both males and females occurs, except from

180-573: The Entoprocta . The fossil record of gnathiferans is poor. There are no known fossil gnathostomulids. Fossils of the extant rotifer genus Habrotrocha are known from Dominican amber dating to the late Eocene , but rotifers are otherwise only known from the Holocene . By contrast, the chaetognath fossil record, while still patchy, includes numerous Paleozoic specimens. Protoconodonts are stem-group chaetognaths. The earliest protoconodonts date to

198-507: The Fortunian age of the Cambrian , and are among the oldest known bilaterians. The enigmatic Cambrian taxon Amiskwia is a gnathiferan and may also be a stem-group chaetognath. The Cambrian ectoparasite Inquicus appears to be a gnathiferan. Gnathifera was named in 1995 to unite gnathostomulids and rotifers. Micrognathozoans were soon added to this grouping. Chaetognaths, long considered

SECTION 10

#1732877249202

216-501: The Latin -fera , “bearing”) is a clade of generally small spiralians characterized by complex jaws made of chitin . It comprises the phyla Gnathostomulida , Rotifera and Micrognathozoa . Chaetognatha has recently been recognised as closely related to the group, with it either being included within Gnathifera or the broader group Chaetognathifera . Gnathiferans include some of

234-453: The asexual animals start producing males. When a male is born, it crawls away from its parent and glues itself to another asexual individual. Once attached, the male produces two dwarf males inside its body, which turns into a hollow pouch. Each of the two dwarf males are about one hundred times smaller than the asexual individual to which they are attached. Their bodies start out with about 200 cells, but this number has been reduced to just 47 by

252-547: The class Bdelloidea . All known gnathiferans are direct developers . Though gnathiferans are included in Spiralia, rotifers and chaetognaths do not exhibit spiral cleavage . Little is known of the development of micrognathozoans. The development of gnathostomulids is poorly known, but they appear to exhibit spiral cleavage. Gnathostomulida Micrognathozoa Chaetognatha Seisonida Acanthocephala Bdelloidea Monogononta Platytrochozoa Gnathifera

270-406: The most abundant phyla. Rotifers are among the most diverse and abundant freshwater animals and chaetognaths are among the most abundant marine plankton. The most distinctive characteristic of gnathiferans is the presence of complex sclerotized mouthparts made of chitin. In most gnathiferans, the anus opens on the dorsal surface of the animal. In micrognathozoans and gnathostomulids, the anus

288-471: 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 their own, new phylum Cycliophora shortly after they were discovered in 1995. This was the first new phylum of multicelled organism to be discovered since the Loricifera in 1983. Symbion

306-450: The time they reach maturity. Thirty-four of the cells form its nervous system, and three more become sensory cells used to help them feel their surroundings. Eight cells becomes mucous glands, which produce mucus that helps them move across the surface. The final two cells form the testes, which make the sperm that fertilize the female's egg. Most of the cells of the dwarf males also lose their nucleus and shrink to almost half their size, which

324-592: Was discovered in 1995 by Reinhardt Kristensen and Peter Funch on the mouthparts of the Norway lobster ( Nephrops norvegicus ). Other, related, species have since been discovered on: The genus is so named because of its commensal relationship with the lobster (a form of symbiosis ) – it feeds on the leftovers from the lobster's own meals. They are peculiar microscopic animals , with no obvious close relatives, which were therefore given their own phylum , called Cycliophora . The phylogenetic position of Symbion

#201798