The Asterozoa are a subphylum in the phylum

Echinodermata An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the s ...

. Characteristics include a star-shaped body and radially divergent axes of symmetry. The subphylum includes the

class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differentl ...

Asteroidea (the starfish), the class

Ophiuroidea Brittle stars, serpent stars, or ophiuroids (; ; referring to the serpent-like arms of the brittle star) are echinoderms in the class Ophiuroidea, closely related to starfish. They crawl across the sea floor using their flexible arms for locomot ...

(the

brittle star Brittle stars, serpent stars, or ophiuroids (; ; referring to the serpent-like arms of the brittle star) are echinoderms in the class Ophiuroidea, closely related to starfish. They crawl across the sea floor using their flexible arms for locomot ...

s and

basket star The Euryalida are an order of brittle stars, which includes large species with either branching arms (called "basket stars") or long and curling arms (called "snake stars"). Characteristics Many of the species in this order have characteristi ...

s), and the extinct order Somasteroidea.

Taxonomy

Asteroidea

Members of the order Asteroidea are characterised by a star-shaped body plan consisting of a central disc and multiple radiating arms. They usually exhibit pentamerous radial symmetry, but some species typically have a symmetry based on a number other than five. The arms have very broad bases and their skeletal support is provided by the calcareous plates called ossicles or ambulacral plates in the body wall. These are joined with muscular and connective tissue giving flexibility. The mouth is on the lower or oral surface, with a corresponding anus on the upper or aboral surface; though the anus is absent in

Paxillosida The Paxillosida are a large order of sea stars. Characteristics Paxillosida adults lack an anus and have no suckers on their tube feet. They do not develop the brachiolaria stage in their early development.Matsubara, M., Komatsu, M., Araki, T., ...

, wherein the mouth serves also as an anus, otherwise leading to a blind gut, similar to their

Ophiuroid Brittle stars, serpent stars, or ophiuroids (; ; referring to the serpent-like arms of the brittle star) are echinoderms in the class Ophiuroidea, closely related to starfish. They crawl across the sea floor using their flexible arms for locomot ...

relatives. Their large coelomic cavity can extend from the disc into the arms. The paired gonads are also located in the arms and release

gamete A gamete (; , ultimately ) is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce ...

s via the gonoducts. The lateral flexion of the arms is limited by how the ambulacral plates are arranged. On the oral surface of the disc and radiating arms are four rows of ambulacral plates which are associated with two or four rows of tube feet, which are used in locomotion. On the upper or aboral surface of the disc is an opening, the

madreporite The madreporite is a light colored calcareous opening used to filter water into the water vascular system of echinoderms. It acts like a pressure-equalizing valve. It is visible as a small red or yellow button-like structure, looking like a sma ...

. This is connected to the

water vascular system The water vascular system is a hydraulic system used by echinoderms, such as sea stars and sea urchins, for locomotion, food and waste transportation, and respiration. The system is composed of canals connecting numerous tube feet. Echinoderms m ...

which assists in respiration and provides hydraulic pressure for the

tube feet Tube feet (technically podia) are small active tubular projections on the oral face of an echinoderm, whether the arms of a starfish, or the undersides of sea urchins, sand dollars and sea cucumbers; they are more discreet though present on br ...

Ophiuroidea

Members of the order Ophiuroidea are characterised by having the gut and internal organs confined to the central disc. The arms are clearly separated from the disc. The tube feet do not have adhesive properties, but serve a sensory function. In all the extant genera, the ambulacral plates are fused in pairs and form joints in the arms, known as vertebrae. In some extinct genera, they are not fused in this way. The ambulacral grooves function as internal epineural canals.

Somasteroidea

All members of the order Somasteroidea (or Stomasteroidea) are extinct. Their first appearance in the fossil record was in the early Ordovician and they had probably died out by the late Devonian. They are similar to the asteroids in that their bodies are flattened dorsoventrally and they have five petaloid arms with broad bases. The ambulacral plates in somasteroids are simple and unspecialized, and the arms were thought to be not flexible and were unable to assist in feeding, but the oral mouth parts were more complex.Somasteroidea
Tree of Life Web Project. Retrieved 2011-09-29. According to the World Register of Marine Species, the Stelleroidea are no longer considered valid. WoRMS states, the name harks back to a previous view of the classification of the echinoderms and its members are now included in Asterozoa. An example for Somasteroidea is '' Cantabrigiaster fezouataensis''.

References

{{Taxonbar, from=Q1407833 Animal subphyla Extant Ordovician first appearances