|The tardigrade Hypsibius dujardini|
|The blue crab Callinectes sapidus , an arthropod|
Tactopoda is a proposed clade of protostome animals that includes the phyla Tardigrada and Euarthropoda, supported by various morphological observations.
A clade, also known as monophyletic group, is a group of organisms that consists of a common ancestor and all its lineal descendants, and represents a single "branch" on the "tree of life". Rather than English, the equivalent Latin term cladus is often used in taxonomical literature.
Protostomia is a clade of animals. Together with the deuterostomes and xenacoelomorpha, its members make up the Bilateria, mostly comprising animals with bilateral symmetry and three germ layers. The major distinctions between deuterostomes and protostomes are found in embryonic development and is based on the embryological origins of the mouth and anus.
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 the deep sea and mud volcanoes, from tropical rain forests to the Antarctic. Tardigrades are among the most resilient animals known, with individual species able to survive extreme conditions that would be rapidly fatal to nearly all other known life forms, such as exposure to extreme temperatures, extreme pressures, air deprivation, radiation, dehydration, and starvation. Tardigrades have even survived after exposure to outer space. About 1,150 known species form the phylum Tardigrada, a part of the superphylum Ecdysozoa. The group includes fossils dating from 530 million years ago, in the Cambrian period.
The competing hypothesis is Arthropoda (= Euarthropoda + Onychophora)
Onychophora, commonly known as velvet worms or more ambiguously as peripatus, is a phylum of elongate, soft-bodied, many-legged panarthropods. In appearance they have variously been compared to worms with legs, caterpillars, and slugs. They prey upon smaller animals such as insects, which they catch by squirting an adhesive slime.
Chasmataspidida is an extinct group of rare chelicerate arthropods. Chasmataspids are probably related to horseshoe crabs (Xiphosura) and/or sea scorpions (Eurypterida). The first species to be discovered were thought to be unusual fossil horseshoe crabs, while later species were often based on specimens initially misidentified as eurypterids. There is some evidence that chasmataspids were present during the late Cambrian and the group is known sporadically in the fossil record through to the mid-Devonian. Chasmataspids are most easily recognised by having an abdomen divided into a short forepart and a longer hindpart comprising nine segments. There is some debate about whether they form a natural group.
Cheloniellida is an unranked taxon of extinct Paleozoic arthropods, currently thought to be a member of the Arachnomorpha and related to the Chelicerata and the Trilobita, while not included in any of those. There are eight included genera, whose fossils are found in marine strata ranging from Ordovician to Devonian in age.
The subphylum Chelicerata constitutes one of the major subdivisions of the phylum Arthropoda. It contains the sea spiders, arachnids, and several extinct lineages, such as the eurypterids.
Xiphosurans are an order of arthropods related to arachnids, or, according to one recent study, actually arachnids. They are sometimes called horseshoe crabs. They first appeared in the Hirnantian. Currently, there are only four living species. They are members of the order Xiphosura, which contains two suborders, Xiphosurida and Synziphosurina.
The class Heterotardigrada includes tardigrades that have cephalic appendages and legs with four separate but similar digits or claws on each. 444 species have been described.
The class Mesotardigrada has only one species, Thermozodium esakii. The animal has six claws of equal length at each foot. This species was recorded in 1937 by Rahm from a hot spring near Nagasaki in Japan. Unfortunately, the locus typicus has been destroyed by an earthquake and subsequent searches for specimens have been unsuccessful. The type specimen has also been lost, so the status of this class is considered dubious.
Arachnomorpha is a subdivision or clade of Arthropoda, comprising the monophyletic group formed by the trilobites, other great appendage arthropods and trilobite-like families, and a diverse sister clade including the chelicerates. Great debate is held on the position of the Pycnogonida, which are currently thought not to be placed in the immediate vicinity of the Chelicerata. Arachnomorpha are considered the sister group to the crustaceans, which are increasingly being accepted as members of the mandibulate clade.
A chordate is an animal constituting the phylum Chordata. During some period of their life cycle, chordates possess a notochord, a dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail: these five anatomical features define this phylum. Chordates are also bilaterally symmetric; and have a coelom, metameric segmentation, and a circulatory system.
Uniramia is a group within the arthropods. In the past this group included the Onychophora, which are now considered a separate category. The group is currently used in a narrower sense.
The lobopodians, members of the informal group Lobopodia are worm-like taxa with stubby legs.
Aysheaia was a genus of Cambrian-aged soft-bodied, caterpillar-shaped fossil organisms with average body lengths of 1–6 cm.
Ecdysozoa is a group of protostome animals, including Arthropoda, Nematoda, and several smaller phyla. They were first defined by Aguinaldo et al. in 1997, based mainly on phylogenetic trees constructed using 18S ribosomal RNA genes. A large study in 2008 by Dunn et al. strongly supported the Ecdysozoa as a clade, that is, a group consisting of a common ancestor and all its descendants.
Panarthropoda is a proposed animal clade combining the extant phyla Arthropoda, Tardigrada and Onychophora, Not all studies support it, but most do, including neuroanatomical, mitogenomic and palaeontological studies. Originally, they were considered to be closely related to the annelids, grouped together as the Articulata, but newer studies place them among the Ecdysozoa.
The Firmicutes are a phylum of bacteria, most of which have gram-positive cell wall structure. A few, however, such as Megasphaera, Pectinatus, Selenomonas and Zymophilus, have a porous pseudo-outer membrane that causes them to stain gram-negative. Scientists once classified the Firmicutes to include all gram-positive bacteria, but have recently defined them to be of a core group of related forms called the low-G+C group, in contrast to the Actinobacteria. They have round cells, called cocci, or rod-like forms (bacillus).
Fibrobacteres is a small bacterial phylum which includes many of the major rumen bacteria, allowing for the degradation of plant-based cellulose in ruminant animals. Members of this phylum were categorized in other phyla. The genus Fibrobacter was removed from the genus Bacteroides in 1988.
The Pentastomida are an enigmatic group of parasitic crustaceans commonly known as tongue worms due to the resemblance of the species of the genus Linguatula to a vertebrate tongue.
In biology, a phylum is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature for algae, fungi, and plants accepts the terms as equivalent. Depending on definitions, the animal kingdom Animalia or Metazoa contains approximately 35 phyla, the plant kingdom Plantae contains about 14, and the fungus kingdom Fungi contains about 8 phyla. Current research in phylogenetics is uncovering the relationships between phyla, which are contained in larger clades, like Ecdysozoa and Embryophyta.
Deuterostomes comprise a superphylum of animals. It is a sister clade of Protostomia, with which it forms the Nephrozoa clade.
An arthropod is an invertebrate animal having an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Euarthropoda, which includes insects, arachnids, myriapods, and crustaceans. The term Arthropoda as originally proposed refers to a proposed grouping of Euarthropods and the phylum Onychophora. Arthropods are characterized by their jointed limbs and cuticle made of chitin, often mineralised with calcium carbonate. The arthropod body plan consists of segments, each with a pair of appendages. The rigid cuticle inhibits growth, so arthropods replace it periodically by moulting. Arthopods are bilaterally symmetrical and their body possesses an external skeleton. Some species have wings.
Cindarella is genus of trilobite-like Cambrian arthropod known from the Chengjiang biota of China. It is classified as a stem group of trilobites in the clade Xandarellida, along with Phytophilaspis, Sinoburius, and Xandarella.
Homalozoa is an obsolete extinct subphylum of Paleozoic Era echinoderms, prehistoric marine invertebrates. They are also referred to as carpoids.
The taxonomy of invertebrates as proposed by Richard C. Brusca and Gary J. Brusca in 2003 is a system of classification with emphasis on the invertebrates, in other words, a way to classify animals, primarily those which have no backbone.
The taxonomy of the animals presented by Hutchins et al. in 2003 in Grzimek's Animal Life Encyclopedia is a system of classification which covers all the metazoans, from phyla to orders.
Varisulca is a proposed basal Podiate taxon. It encompasses several lineages of heterotrophic protists, most notably the ancyromonads (planomonads), collodictyonids (diphylleids), rigifilids and Mantamonas. Recent evidence suggests that the latter three are closely related to each other, forming a clade called CRuMs, but that this is unlikely to be specifically related to ancyromonads
The Artiopoda is a grouping of extinct arthropods that includes trilobites and their close relatives. It was erected by Hou and Bergström in 1997 to encompass a wide diversity of arthropods that would traditionally have been assigned to the Trilobitomorpha. Hou and Bergström used the name Lamellipedia as a superclass to replace Trilobitomorpha that was originally erected at the subphylum level, which they considered inappropriate.
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