In biology , a taxon ( back-formation from taxonomy ; pl. : taxa ) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and given a particular ranking , especially if and when it is accepted or becomes established. It is very common, however, for taxonomists to remain at odds over what belongs to a taxon and the criteria used for inclusion, especially in the context of rank-based (" Linnaean ") nomenclature (much less so under phylogenetic nomenclature ). If a taxon is given a formal scientific name , its use is then governed by one of the nomenclature codes specifying which scientific name is correct for a particular grouping.
23-448: See text The order Cephalaspidea , also known as the headshield slugs and bubble snails , is a major taxon of sea slugs and bubble snails, marine gastropod mollusks within the larger clade Euopisthobranchia . Bubble shells is another common name for these families of marine gastropods, some of which have thin bubble-like shells. This clade contains more than 600 species. Members of this worldwide clade used to be considered
46-469: A "good" or "useful" taxon is commonly taken to be one that reflects evolutionary relationships . Many modern systematists, such as advocates of phylogenetic nomenclature , use cladistic methods that require taxa to be monophyletic (all descendants of some ancestor). Therefore, their basic unit, the clade , is equivalent to the taxon, assuming that taxa should reflect evolutionary relationships. Similarly, among those contemporary taxonomists working with
69-452: A dark brown pit at the base of the right rhinophore . The taxonomy of the shelled cephalaspideans, the bubble snails, like that of many shelled mollusks, used to be based very simply on shell characteristics. But because there are some similarities in shell morphology throughout this group, more recently taxonomists have taken other anatomical characteristics into consideration, such as the radula, gizzard, penis, and Hancock's organ. In 2015
92-467: A narrow set of ranks is challenged by users of cladistics ; for example, the mere 10 ranks traditionally used between animal families (governed by the International Code of Zoological Nomenclature (ICZN)) and animal phyla (usually the highest relevant rank in taxonomic work) often cannot adequately represent the evolutionary history as more about a lineage's phylogeny becomes known. In addition,
115-609: A new study based on molecular phylogenetics has changed significantly the taxonomy of the Cephalaspidea. The monophyly of the Cephalaspidea was confirmed, but the families Cylichnidae, Diaphanidae, Haminoeidae, Philinidae, and Retusidae were found non-monophyletic. This had led to the creation of new families (( Alacuppidae , Colinatydidae , Colpodaspididae , Mnestiidae , Philinorbidae ) ) and one new genus ( Alacuppa ). Two family names ( Acteocinidae , Laonidae ) and two genera ( Laona , Philinorbis ) are reinstated as valid In
138-427: A rank above, the prefix sub- indicates a rank below. In zoology , the prefix infra- indicates a rank below sub- . For instance, among the additional ranks of class are superclass, subclass and infraclass. Rank is relative, and restricted to a particular systematic schema. For example, liverworts have been grouped, in various systems of classification, as a family, order, class, or division (phylum). The use of
161-468: A unit-based system of biological classification was first made widely available in 1805 in the introduction of Jean-Baptiste Lamarck 's Flore françoise , and Augustin Pyramus de Candolle 's Principes élémentaires de botanique . Lamarck set out a system for the "natural classification" of plants. Since then, systematists continue to construct accurate classifications encompassing the diversity of life; today,
184-487: Is arranged as follows (there are listed genera under molecular analysis; not analyzed families are under " incertae sedis "): Architectibranchia Haszprunar, 1985 Runcinacea Burn, 1963 Cephalaspidea Fischer, 1887 – This means Cephalaspidea sensu stricto Subsequently, Malaquias (2010) moved Bullacta exarata (formerly the only member of Bullactidae) into the family Haminoeidae . Jörger et al. (2010) moved Cephalaspidea sensu stricto and Runcinacea into
207-742: Is ovoid, thin and translucent. It may be smooth or have spiral grooves (striae). The umbilical apex is sunken or enclosed and no longer visible. Large body whorl with fine spiral striations. Smooth columella. The thin outer lip of the aperture extends beyond the apex of the shell and is thus longer than the body whorl. The aperture narrows posteriorly and is wider anteriorly. These snails are herbivorous . Their diet consists of various kinds of green algae . They can survive in brackish water. They are hermaphrodites . Their eggs are deposited in round or oval jellylike strings, attached to eelgrass or sand. Atys naucum and Atys cylindricum are known to show biological fluorescence . Genera within
230-517: The International Code of Zoological Nomenclature (1999) defines a A taxon can be assigned a taxonomic rank , usually (but not necessarily) when it is given a formal name. " Phylum " applies formally to any biological domain , but traditionally it was always used for animals, whereas "division" was traditionally often used for plants , fungi , etc. A prefix is used to indicate a ranking of lesser importance. The prefix super- indicates
253-798: The Euopisthobranchia and they confirmed the placement of Acteonoidea within the Lower Heterobranchia . All families of Architectibranchia were already within the Lower Heterobranchia in the taxonomy of Bouchet & Rocroi, except for the Notodiaphanidae , which has been placed in the Lower Heterobranchia since 2010, in order that the Architectibranchia can be considered to be monophyletic . The publication by Oskars T.R., Bouchet P. & Malaquias M.A. (2015) . A new phylogeny of
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#1733085826675276-498: The taxonomy of Bouchet & Rocroi (2005), the clade Cephalaspidea is arranged as follows: The superfamily Acteonoidea has been included into the new Informal Group "Lower Heterobranchia" and the superfamily Cylindrobulloidea becomes part of the Group Cylindrobullida. Malaquias et al. (2009) have rearranged taxonomy of Cephalaspidea sensu lato : The taxonomy of Cephalaspidea sensu lato by Malaquias et al. (2009)
299-501: The Cephalaspidea (Gastropoda: Heterobranchia) based on expanded taxon sampling and gene markers. in the journal Molecular Phylogenetics and Evolution. 89 came to the following conclusion, with the creation of new families The superfamily Bulloidea was not supported in the Bayesian phylogenetic hypothesis and Diaphanoidea was found polyphyletic . The superfamilies Haminoeoidea and Philinoidea were accepted. The composition of each of
322-601: The application of names to clades . Many cladists do not see any need to depart from traditional nomenclature as governed by the ICZN, International Code of Nomenclature for algae, fungi, and plants , etc. Haminoeidae Haminoeidae , commonly known as the haminoeid bubble snail family , is a taxonomic family of sea snails, marine opisthobranch gastropod mollusks in the superfamily Haminoeoidea . The name of this family has long been controversial, and used to be Atyidae or Atydidae. Another, but incorrect, spelling
345-407: The brightly colored genus Chelidonura also have well-developed eyes on the anterior end of the head and bundles of sensory cilia around the mouth. With these cilia they are able to track their prey by following the victim's mucous trail. The Hancock's organ is a chemosensory organ situated between the foot and the headshield. It plays a role in olfactory and sensory detection. It is visible as
368-511: The class rank is quite often not an evolutionary but a phenetic or paraphyletic group and as opposed to those ranks governed by the ICZN (family-level, genus-level and species -level taxa), can usually not be made monophyletic by exchanging the taxa contained therein. This has given rise to phylogenetic taxonomy and the ongoing development of the PhyloCode , which has been proposed as a new alternative to replace Linnean classification and govern
391-481: The most ancestral of the opisthobranchs , but now they are considered as derived and specialized members of the Euthyneura Spengel, 1881. Headshield slugs are the most morphologically diverse group of all the opisthobranchs. The vast majority possess a shell, although it may be reduced or internal. They have a well-developed headshield, a characteristic broadening at the head, which is used to plow beneath
414-429: The shore in tidepools. These are colorful snails, that can partially take the color of the sea floor. Their large cephalic shield is rounded at the front, but deeply lobed behind. The mantle protrudes behind the shell. The shell is partially or completely enfolded by lateral parapodial (=fleshy winglike outgrowths) lobes. Their shell varies in size according to the species, from 3 mm to 30 mm. The shell
437-608: The superfamilies was drastically rearranged. Taxon Initial attempts at classifying and ordering organisms (plants and animals) were presumably set forth in prehistoric times by hunter-gatherers, as suggested by the fairly sophisticated folk taxonomies. Much later, Aristotle, and later still, European scientists, like Magnol , Tournefort and Carl Linnaeus 's system in Systema Naturae , 10th edition (1758), , as well as an unpublished work by Bernard and Antoine Laurent de Jussieu , contributed to this field. The idea of
460-474: The surface of the sand. This headshield prevents the sand entering the mantle cavity. There is a muscular foot with or without parapodia (fleshy wing-like flaps). Headshield slugs often live just beneath the surface of the sand and can also be seen crawling on rocks. They have well-developed sensory structures to detect prey, which may be other opisthobranchs, polychaetes or bristleworms and foraminiferans . Several species are voracious carnivores. Members of
483-488: The traditional Linnean (binomial) nomenclature, few propose taxa they know to be paraphyletic . An example of a long-established taxon that is not also a clade is the class Reptilia , the reptiles; birds and mammals are the descendants of animals traditionally classed as reptiles, but neither is included in the Reptilia (birds are traditionally placed in the class Aves , and mammals in the class Mammalia ). The term taxon
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#1733085826675506-511: Was Haminaeidae (See (ICZN) 2000. Opinion 1942). A number of genera have been proposed for this family, but the species are hard to identify (or sometimes impossible to identify) by looking only at the external characteristics. Until the internal anatomy of 'wet' specimens has been fully described, the status of many of the genera listed here is uncertain. These bubble snails occur in all warm or temperate seas. These are sand dwellers or they live on muddy bottoms, in bays, estuaries, and close to
529-580: Was first used in 1926 by Adolf Meyer-Abich for animal groups, as a back-formation from the word taxonomy ; the word taxonomy had been coined a century before from the Greek components τάξις ( táxis ), meaning "arrangement", and νόμος ( nómos ), meaning " method ". For plants, it was proposed by Herman Johannes Lam in 1948, and it was adopted at the VII International Botanical Congress , held in 1950. The glossary of
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