In biological classification , class ( Latin : classis ) is a taxonomic rank , as well as a taxonomic unit, a taxon , in that rank. It is a group of related taxonomic orders. Other well-known ranks in descending order of size are life , domain , kingdom , phylum , order , family , genus , and species , with class ranking between phylum and order.
12-497: Octocorallia (also known as Alcyonaria ) is a class of Anthozoa comprising over 3,000 species of marine organisms formed of colonial polyps with 8-fold symmetry. It includes the blue coral , soft corals , sea pens , and gorgonians (sea fans and sea whips) within three orders: Alcyonacea, Helioporacea, and Pennatulacea. These organisms have an internal skeleton secreted by mesoglea and polyps with eight tentacles and eight mesentaries. As with all cnidarians these organisms have
24-495: A top-level genus (genus summum) – was first introduced by French botanist Joseph Pitton de Tournefort in the classification of plants that appeared in his Eléments de botanique of 1694. Insofar as a general definition of a class is available, it has historically been conceived as embracing taxa that combine a distinct grade of organization—i.e. a 'level of complexity', measured in terms of how differentiated their organ systems are into distinct regions or sub-organs—with
36-546: A complex life cycle including a motile phase when they are considered plankton and later characteristic sessile phase. Octocorals have existed at least since the Ordovician period, as shown by Maurits Lindström 's findings in the 1970s. Recent work suggesting that the Cambrian Pywackia may represent a Cambrian octocoral is disputed. Octocorals resemble the stony corals in general appearance and in
48-449: A continuous epidermis and perforated by numerous tiny channels. The channels interconnect the gastrovascular cavities of the polyps, allowing water and nutrients to flow freely between all the members of the colony. The skeletal material, called coenenchyme , is composed of living tissue secreted by numerous wandering amoebocytes . Although it is generally soft, in many species it is reinforced with calcareous or horny material. The polyp
60-434: A distinct type of construction, which is to say a particular layout of organ systems. This said, the composition of each class is ultimately determined by the subjective judgment of taxonomists . In the first edition of his Systema Naturae (1735), Carl Linnaeus divided all three of his kingdoms of nature ( minerals , plants , and animals ) into classes. Only in the animal kingdom are Linnaeus's classes similar to
72-454: Is found in 32 genera, a trait estimated to have evolved 540 million years ago, the earliest timing of emergence of bioluminescence in the marine environment. Octocorallia is considered to be monophyletic , meaning that all contained species are descended from a common ancestor, but the relationships between subdivisions are not well known. The sea pens (Pennatulacea) and blue coral (Helioporacea) continue to be assigned separate orders , whereas
84-464: Is largely embedded within the colonial skeleton, with only the uppermost surface, including the tentacles and mouth, projecting about the surface. The mouth is slit-like, with a single ciliated groove, or siphonoglyph , at one side to help control water flow. It opens into a tubular pharynx that projects down into a gastrovascular cavity that occupies the hollow interior. The pharynx is surrounded by eight radial partitions, or mesenteries , that divide
96-450: The classes used today; his classes and orders of plants were never intended to represent natural groups, but rather to provide a convenient "artificial key" according to his Systema Sexuale , largely based on the arrangement of flowers. In botany, classes are now rarely discussed. Since the first publication of the APG system in 1998, which proposed a taxonomy of the flowering plants up to
108-540: The current order Alcyonacea was historically represented by four orders: Alcyonacea, Gorgonacea, Stolonifera and Telestacea. The following taxa are unplaced within Octocorallia according to the World Register of Marine Species as of April 2024: Families: Genera: Class (biology) The class as a distinct rank of biological classification having its own distinctive name – and not just called
120-503: The level of orders, many sources have preferred to treat ranks higher than orders as informal clades . Where formal ranks have been assigned, the ranks have been reduced to a very much lower level, e.g. class Equisitopsida for the land plants, with the major divisions within the class assigned to subclasses and superorders. The class was considered the highest level of the taxonomic hierarchy until George Cuvier 's embranchements , first called Phyla by Ernst Haeckel , were introduced in
132-409: The size of their polyps, but lack the distinctive stony skeleton. Also unlike the stony corals, each polyp has only eight tentacles, each of which is feather-like in shape, with numerous side-branches, or pinnules . Octocorals are colonial organisms, with numerous tiny polyps embedded in a soft matrix that forms the visible structure of the colony. The matrix is composed of mesogleal tissue, lined by
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#1732773239270144-429: The upper part of the gastrovascular cavity into chambers, one of which connects to the hollow space inside each tentacle. The gonads are located near the base of each mesentery. Octocorals have high phenotypic plasticity from adapting to dynamic environments where temperature, pH, and other parameters are in constant flux and have shown high recruitment rates post-die-off events caused by el nino events. Bioluminescence
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