Asterozoa is a subphylum in the phylum

Phylum

In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....

 Echinodermata. Characteristics include a star-shaped body and radially divergent axes of symmetry. The phylum includes the two orders

Order (biology)

In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

 Asteroidea, the starfish, and Ophiuroidea, the brittle star

Brittle star

Brittle stars or ophiuroids are echinoderms in the class Ophiuroidea closely related to starfish. They crawl across the seafloor using their flexible arms for locomotion. The ophiuroids generally have five long slender, whip-like arms which may reach up to in length on the largest specimens...

s and basket star

Basket star

Basket stars are a group of brittle stars. They are treated as a suborder Euryalina or order Euryalida. Many of them have characteristic many-branched arms. They generally live in deep sea habitats.The life span in the wild is up to 35 years. They weigh around 11 lbs, or 5 kg...

s and perhaps the extinct order Somasteroidea.

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. It serves also as an anus and leads to a blind gut. This large coelomic cavity can extend from the disc into the arms. The paired gonad

Gonad

The gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...

s are also located in the arms and release gamete

Gamete

A gamete is a cell that fuses with another cell during fertilization in organisms that reproduce sexually...

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, there 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 a madreporite

Madreporite

The madreporite is a lightcolored calcerous 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 small wart, on the aboral surface of the central disk of a...

. This is connected to the water vascular system

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...

 which assists in respiration and provides hydraulic pressure for the tube feet

Tube feet

Tube feet are the many small tubular projections found most famously on the oral face of a sea star's arms, but are characteristic of the water vascular system of the echinoderm phylum which also includes sea urchins, sand dollars and sea cucumbers and many other sea creatures.Tube feet function in...

.

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

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

, 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 dorso-ventrally and they have five petaloid arms with broad bases. The ambulacral plates in somasteroids are simple and unspecialised and it is thought that the arms were not flexible and were unable to assist in feeding but the oral mouth part were more complex. According to the World Register of Marine Species, Stelleroidea is no longer considered valid. WoRMS states that the name harks back to a previous view of the classification of the echinoderms and that its members are now included in Asterozoa.