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17-410: Acoelomorpha is a subphylum of very simple and small soft-bodied animals with planula -like features which live in marine or brackish waters . They usually live between grains of sediment , swimming as plankton , or crawling on other organisms, such as algae and corals . With the exception of two acoel freshwater species, all known acoelomorphs are marine. The term "acoelomorph" derives from

34-572: A group of botanists including Harold Charles Bold , Arthur Cronquist and Lynn Margulis proposed replacing the term "division" with "phylum" in botanical nomenclature, arguing that maintaining different terms for the same taxonomic rank across biological kingdoms created unnecessary confusion. This was particularly problematic for unicellular eukaryotes , where heterotrophic organisms were classified under zoological nomenclature (using "phylum") while autotrophic organisms fell under botanical nomenclature (using "division"). Their proposal to standardise

51-400: A simpler anatomy, not even having a gut. Like flatworms, they have no circulatory or respiratory systems, but they also lack an excretory system. They lack body cavities (acoelomate structure), a hindgut or an anus. The epidermal cells of acoelomorphs are unable to proliferate, a feature that is only shared with rhabditophoran flatworms and was for some time considered a strong evidence for

68-470: A subphylum of phylum Angiospermae and vertebrates as a subphylum of phylum Chordata . Subphylum is: Where convenient, subphyla in turn can be divided into infraphyla ; in turn such an infraphylum also would be superordinate to any classes or superclasses in the hierarchy . Not all fauna phyla are divided into subphyla. Those that are include: Examples of infraphyla include the Mycetozoa ,

85-481: The Ancient Greek words ἀ ( a ), the alpha privative , expressing negation or absence, κοιλία ( koilía ), meaning "cavity", and μορφή ( morphḗ ), meaning "form". This refers to the fact that acoelomorphs have a structure lacking a fluid-filled body cavity. The subphylum Acoelomorpha is divided into two classes. There are at least 408 described species, with a majority of these falling within

102-523: The Crucimusculata infraorder in Acoela. The soft bodies of acoelomorphs and the lack of some of the key bilaterian traits make them difficult to classify. Traditionally, based on phenotypic features , acoelomorphs were considered to belong to the phylum Platyhelminthes , which was long seen as the sister group to all other bilaterian phyla. However, a series of molecular phylogenetics studies at

119-694: The Gnathostomata and the Agnatha . Subdivision (rank) Division is a taxonomic rank in biological classification that is used differently in zoology and in botany. In botany and mycology , division refers to a rank equivalent to phylum . The use of either term is allowed under the International Code of Botanical Nomenclature , and both are commonly used in scientific literature. The main Divisions of land plants are

136-731: The International Committee on Taxonomy of Viruses (ICTV) formalised a 15-rank hierarchical classification system, ranging from the highest rank " realm " (rather than domain) down through the familiar ranks, notably using "phylum" rather than "division" at this level. This change aligns virus taxonomy more closely with zoological nomenclature and reflects a growing recognition that maintaining different terms (phylum vs. division) for equivalent ranks creates unnecessary complexity, particularly when classifying organisms like unicellular eukaryotes that may be treated under different codes depending on their nutritional mode. Under this system,

153-542: The Marchantiophyta (liverworts), Anthocerotophyta (hornworts), Bryophyta (mosses), Filicophyta (ferns), Sphenophyta (horsetails), Cycadophyta (cycads), Ginkgophyta (ginkgo)s, Pinophyta (conifers), Gnetophyta (gnetophytes), and the Magnoliophyta (Angiosperms, flowering plants). The Magnoliophyta now dominate terrestrial ecosystems , comprising 80% of vascular plant species. In zoology ,

170-555: The 1980s. In 1985, Carl Woese and colleagues identified ten major groups of eubacteria through oligonucleotide signature analysis, noting that these groupings were "appropriately termed eubacterial Phyla or Divisions." This work provided early molecular evidence for the equivalence of bacterial divisions with phyla and helped establish a phylogenetic basis for high-level bacterial classification. The traditional term "division" in botanical and mycological taxonomy has been increasingly challenged by modern classification systems. In 2020,

187-399: The female reproductive system. Instead, gametes are produced from the mesenchymal cells that fill the body between the epidermis and the digestive vacuole. Subphylum The taxonomic rank of " subdivision " in fungi and plant taxonomy is equivalent to "subphylum" in zoological taxonomy. Some plant taxonomists have also used the rank of subphylum, for instance monocotyledons as

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204-548: The hinge between the 20th and 21st centuries demonstrated that they are fast-evolving organisms not closely related to platyhelminthes, therefore involving the polyphyly of flatworms. Actually, Acoelomorpha appeared to constitute a separate, deep-branching phylum, kingpin of bilaterian evolution. Yet their evolutionary affinities remain enigmatic as they might be the sister-group either to all other bilateral animals or to all deuterostomes. Resolving this debate would indicate whether acoelomorphs are simple or simplified. If they are

221-465: The position of Acoelomorpha within Platyhelminthes. In both groups, the epidermis is renewed from mesodermal stem cells. The nervous system of acoelomorphs is formed by a set of longitudinal nerve bundles beneath the ciliated epidermis. Close to the anterior end, these bundles are united by a ring commissure, but do not form a true brain, although it is hypothesized that such organization was

238-457: The precursor of the cephalization of the nerve system in more derived bilaterians. After decapitation, such a "brain" (rather, a cerebroid ganglion ) regenerates in a few weeks. The sensory organs include a statocyst – which presumably helps them orient to gravity –, and, in some cases, ancestral pigment-spot ocelli capable of detecting light. Acoelomorphs are simultaneous hermaphrodites , but have no gonads and no ducts associated with

255-418: The sister group to Bilateria , it would point to a simple body plan for the first bilaterian. Alternatively, if acoelomorphs are related to deuterostomes , this would imply that their organisation is the result of secondary simplification. In addition, comparative analyses of morphological, developmental, and molecular characters raised two points. Acoelomorphs resemble flatworms in many respects, but have

272-585: The term division is applied to an optional rank subordinate to the infraclass and superordinate to the cohort . A widely used classification (e.g. Carroll 1988 ) recognises teleost fishes as a Division Teleostei within Class Actinopterygii (the ray-finned fishes). Less commonly (as in Milner 1988 ), living tetrapods are ranked as Divisions Amphibia and Amniota within the clade of vertebrates with fleshy limbs ( Sarcopterygii ). In 1978,

289-529: The terminology aimed to reflect the growing scientific appreciation for the unity of all organisms. They proposed updating the International Code of Botanical Nomenclature to use "phylum" and "subphylum" throughout, while maintaining that names originally published as divisions would be treated as if they had been published as phyla. The use of molecular methods, particularly 16S ribosomal RNA analysis, helped establish major bacterial divisions in

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