| Eleutherozoa Temporal range: | |
|---|---|
 | |
| Pisaster ochraceaus | |
Scientific classification  | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Echinodermata |
| Subphylum: | Eleutherozoa Bather, 1900 |
| Classes [1] | |
| |
Eleutherozoa is a proposed subphylum of echinoderms. They are mobile animals with the mouth directed towards the substrate. They usually have a madreporite, tube feet, and moveable spines of some sort. It includes all living echinoderms except for crinoids.
There are 2 main competing hypotheses about the internal subdivision, both about equally well supported by both molecular and morphological data. They differ in their placement of the Ophiuroidea (brittle stars), and are named accordingly.
The "Cryptosyringida" hypothesis posits that the "sea-star" morphology is plesiomorphic for Eleutherozoa as a whole, and that starfish (Asteroidea) and brittle stars are not very closely related, the latter forming the clade Cryptosyringida together with the Echinozoa. The "Asterozoa" hypothesis, on the other hand, implies that the "sea-star" arms of starfish and brittle stars, as well as the rounded shape of Echinozoa, all evolved independently from an ancestor of unknown morphology, but that each "armed" and "rounded" lineage is strictly monophyletic. Too little is known of the basal eleutherozoans and echinoderms to be able to firmly decide for or against any of these hypotheses at present. [2]
The Asterozoa would have to be ranked as a superclass or treated as an unranked clade between the Cryptosyringida and the Eleutherozoa, depending on whether the "Asterozoa" or "Cryptosyringida" hypothesis eventually turns out to be correct.
Some old research favours the following classification: [3] [4]
| Echinodermata |
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Modern ananlysis favours this cladogram: [5] [6]
| Echinodermata |
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A chordate is a deuterostomic animal belonging to the phylum Chordata. All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics (synapomorphies) that distinguish them from other taxa. These five synapomorphies are a notochord, a hollow dorsal nerve cord, an endostyle or thyroid, pharyngeal slits, and a post-anal tail. The name "chordate" comes from the first of these synapomorphies, the notochord, which plays a significant role in chordate body plan structuring and movements. Chordates are also bilaterally symmetric, have a coelom, possess a closed circulatory system, and exhibit metameric segmentation.
An echinoderm is any deuterostomal animal of the phylum Echinodermata, which includes starfish, brittle stars, sea urchins, sand dollars and sea cucumbers, as well as the sessile sea lilies or "stone lilies". While bilaterally symmetrical as larvae, as adults echinoderms are recognisable by their usually five-pointed radial symmetry, and are found on the sea bed at every ocean depth from the intertidal zone to the abyssal zone. The phylum contains about 7,600 living species, making it the second-largest group of deuterostomes after the chordates, as well as the largest marine-only phylum. The first definitive echinoderms appeared near the start of the Cambrian.
Crinoids are marine invertebrates that make up the class Crinoidea. Crinoids that remain attached to the sea floor by a stalk in their adult form are commonly called sea lilies, while the unstalked forms, called feather stars or comatulids, are members of the largest crinoid order, Comatulida. Crinoids are echinoderms in the phylum Echinodermata, which also includes the starfish, brittle stars, sea urchins and sea cucumbers. They live in both shallow water and in depths as great as 9,000 meters (30,000 ft).
Bilateria is a large clade or infrakingdom of animals called bilaterians, characterized by bilateral symmetry during embryonic development. This means their body plans are laid around a longitudinal axis with a front and a rear end, as well as a left–right–symmetrical belly (ventral) and back (dorsal) surface. Nearly all bilaterians maintain a bilaterally symmetrical body as adults; the most notable exception is the echinoderms, which extend to pentaradial symmetry as adults, but are only bilaterally symmetrical as an embryo. Cephalization is also a characteristic feature among most bilaterians, where the special sense organs and central nerve ganglia become concentrated at the front/rostral end.
Sea urchins or urchins, alternatively known as sea hedgehogs, are typically spiny, globular animals, echinoderms in the class Echinoidea. About 950 species live on the seabed, inhabiting all oceans and depth zones from the intertidal to 5,000 metres. Their tests are round and spiny, typically from 3 to 10 cm across. Sea urchins move slowly, crawling with their tube feet, and sometimes pushing themselves with their spines. They feed primarily on algae but also eat slow-moving or sessile animals. Their predators include sea otters, starfish, wolf eels, and triggerfish.
Ecdysozoa is a group of protostome animals, including Arthropoda, Nematoda, and several smaller phyla. The grouping of these animal phyla into a single clade was first proposed by Eernisse et al. (1992) based on a phylogenetic analysis of 141 morphological characters of ultrastructural and embryological phenotypes. This clade, that is, a group consisting of a common ancestor and all its descendants, was formally named by Aguinaldo et al. in 1997, based mainly on phylogenetic trees constructed using 18S ribosomal RNA genes.
Starfish or sea stars are star-shaped echinoderms belonging to the class Asteroidea. Common usage frequently finds these names being also applied to ophiuroids, which are correctly referred to as brittle stars or basket stars. Starfish are also known as asteroids due to being in the class Asteroidea. About 1,900 species of starfish live on the seabed in all the world's oceans, from warm, tropical zones to frigid, polar regions. They are found from the intertidal zone down to abyssal depths, at 6,000 m (20,000 ft) below the surface.
Eumetazoa, also known as diploblasts, Epitheliozoa or Histozoa, are a proposed basal animal clade as a sister group of Porifera (sponges). The basal eumetazoan clades are the Ctenophora and the ParaHoxozoa. Placozoa is now also seen as a eumetazoan in the ParaHoxozoa. The competing hypothesis is the Myriazoa clade.
Brittle stars, serpent stars, or ophiuroids are echinoderms in the class Ophiuroidea, closely related to starfish. They crawl across the sea floor using their flexible arms for locomotion. The ophiuroids generally have five long, slender, whip-like arms which may reach up to 60 cm (24 in) in length on the largest specimens.
Tube feet are small active tubular projections on the oral face of an echinoderm, such as the arms of a starfish, or the undersides of sea urchins, sand dollars and sea cucumbers; they are more discreet though present on brittle stars, and have only a feeding function in feather stars. They are part of the water vascular system.
Dipleurula is a hypothetical larva of the ancestral echinoderm. It represents the type of basis of all larval forms of, at least, the eleutherozoans, where the starfish, sea urchins, sea cucumbers and brittle stars belong. The dipleurula is a bilaterally symmetrical, ciliated echinoderm larva.
The stylophorans are an extinct, possibly polyphyletic group allied to the Paleozoic Era echinoderms, comprising the prehistoric cornutes and mitrates. It is synonymous with the subphylum Calcichordata. Their unusual appearances have led to a variety of very different reconstructions of their anatomy, how they lived, and their relationships to other organisms.
Ambulacraria, or Coelomopora, is a clade of invertebrate phyla that includes echinoderms and hemichordates; a member of this group is called an ambulacrarian. Phylogenetic analysis suggests the echinoderms and hemichordates separated around 533 million years ago. The Ambulacraria are part of the deuterostomes, a clade that also includes the many Chordata, and the few extinct species belonging to the Vetulicolia.
Deuterostomes are bilaterian animals of the superphylum Deuterostomia, typically characterized by their anus forming before the mouth during embryonic development. Deuterostomia is further divided into 4 phyla: Chordata, Echinodermata, Hemichordata, and the extinct Vetulicolia known from Cambrian fossils. The extinct clade Cambroernida is also thought to be a member of Deuterostomia.
The Spiralia are a morphologically diverse clade of protostome animals, including within their number the molluscs, annelids, platyhelminths and other taxa. The term Spiralia is applied to those phyla that exhibit canonical spiral cleavage, a pattern of early development found in most members of the Lophotrochozoa.
Sea star wasting disease or starfish wasting syndrome is a disease of starfish and several other echinoderms that appears sporadically, causing mass mortality of those affected. There are approximately 40 species of sea stars that have been affected by this disease. At least 20 of these species were on the Northwestern coast of Mexico to Alaska. The disease seems to be associated with increased water temperatures in some locales, but not others. It starts with the emergence of lesions, followed by body fragmentation and death. In 2014 it was suggested that the disease is associated with a single-stranded DNA virus now known as the sea star-associated densovirus (SSaDV). However, this hypothesis was refuted by recent research in 2018 and 2020. Sea star wasting disease is still not fully understood.
The echinoderm and flatworm mitochondrial code is a genetic code used by the mitochondria of certain echinoderm and flatworm species.
Catch connective tissue is a kind of connective tissue found in echinoderms which can change its mechanical properties in a few seconds or minutes through nervous control rather than by muscular means.
Maria Byrne is an Australian marine biologist, and professor of marine and developmental biology at the University of Sydney and a member of the Sydney Environment Institute. She spent 12 years as director of the university's research station on One Tree Island.
Villebrunaster is an extinct genus of starfish-like animal belonging to Asterozoa that lived around 480 million years ago during Early Ordovician Period in modern-day southern France and Morocco. As of 2022, it contains two species, namely V. thorali and V. fezouataensis. V. thorali was described in 1951 and V. fezouataensis was described in 2021. Villebrunaster represents one of the oldest members of asterozoans, and perhaps, according to a description in 2021, the earliest divergent stem-group of Asterozoa.
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