Amongst the brittle stars, six-armed species such as Ophiothela danae, Ophiactis savignyi, and Ophionotus hexactis exist, and Ophiacantha vivipara often has more than six.

[15] Many crinoids and some seastars are symmetrical in multiples of the basic five; starfish such as Labidiaster annulatus possess up to fifty arms, while the sea-lily Comaster schlegelii has two hundred.

[21] The epidermis contains pigment cells that provide the often vivid colours of echinoderms, which include deep red, stripes of black and white, and intense purple.

The reaction can happen quickly: the sea urchin Centrostephanus longispinus changes colour in just fifty minutes when exposed to light.

This tissue enables a starfish to go from moving flexibly around the seabed to becoming rigid while prying open a bivalve mollusc or preventing itself from being extracted from a crevice.

[24][25] Echinoderms possess a unique water vascular system, a network of fluid-filled canals modified from the coelom (body cavity) that function in gas exchange, feeding, sensory reception and locomotion.

The water vascular system assists with the distribution of nutrients throughout the animal's body; it is most visible in the tube feet which can be extended or contracted by the redistribution of fluid between the foot and the internal ampulla.

[29] The arrangement in crinoids is similar to that in asteroids, but the tube feet lack suckers and are used in a back-and-forth wafting motion to pass food particles captured by the arms towards the central mouth.

Starfish are mostly carnivorous and have a mouth, oesophagus, two-part stomach, intestine and rectum, with the anus located in the centre of the aboral body surface.

Boluses of mucus-trapped food are passed to the mouth, which is linked to the anus by a loop consisting of a short oesophagus and longer intestine.

All classes possess a type of phagocytic amebocyte, which engulf invading particles and infected cells, aggregate or clot, and may be involved in cytotoxicity.

[43][44] The coelomocytes secrete antimicrobial peptides against bacteria, and have a set of lectins and complement proteins as part of an innate immune system that is still being characterised.

[45] Echinoderms have a simple radial nervous system that consists of a modified nerve net of interconnected neurons with no central brain, although some do possess ganglia.

Sea urchins have no particular sense organs but do have statocysts that assist in gravitational orientation, and they too have sensory cells in their epidermis, particularly in the tube feet, spines and pedicellariae.

[50][51][52] During periods when they have lost their digestive tracts, sea cucumbers live off stored nutrients and absorb dissolved organic matter directly from the water.

In a very small number of species, the eggs are retained in the coelom where they develop viviparously, later emerging through ruptures in the body wall.

[61] In some crinoids, the embryos develop in special breeding bags, where the eggs are held until sperm released by a male happens to find them.

New larvae can develop from the preoral hood (a mound like structure above the mouth), the side body wall, the postero-lateral arms, or their rear ends.

Small fish landing on the upper surface may be captured by pedicilaria and dead animal matter may be scavenged but the main prey items are living invertebrates, mostly bivalve molluscs.

Larger starfish prey on smaller ones; the great quantity of eggs and larva that they produce form part of the zooplankton, consumed by many marine creatures.

[96] Antipredator defences include the presence of spines, toxins (inherent or delivered through the tube feet), and the discharge of sticky entangling threads by sea cucumbers.

Although most echinoderm spines are blunt, those of the crown-of-thorns starfish are long and sharp and can cause a painful puncture wound as the epithelium covering them contains a toxin.

Some sea cucumbers have a cluster of cuvierian tubules which can be ejected as long sticky threads from their anus to entangle and permanently disable an attacker.

Coral reefs are also bored into in this way, but the rate of accretion of carbonate material is often greater than the erosion produced by the sea urchin.

In 1983, for example, the mass mortality of the tropical sea urchin Diadema antillarum in the Caribbean caused a change from a coral-dominated reef system to an alga-dominated one.

The lack of a consensus cladistic phylogeny incorporating extinct echinoderm groups has resulted in the continued use of terms from Linnaean taxonomies, even when the named taxa are known to be paraphyletic and/or polyphyletic.

Supporters of pentaradiality as an initial condition of the phylum note that radial forms are the first uncontested echinoderms to appear in the fossil record.

Arkarua† Helicoplacoidea† Edrioasteroidea† Lepidocystiodea† some Eocrinoids† some Eocrinoids† some Eocrinoids† Homostelea† (Cincta) Ctenocystoidea† Homoiostelea† (Soluta) Diploporita† Rhombifera† Blastoidea† Crinoidea Stylophora† Ophiuroidea Asteroidea Echinoidea Ophiocistioidea† Holothuroidea In this theory, the controversial[144] Ediacaran fossil Arkarua is tentatively placed as the sister to all other echinoderms.

[161] Popular species include the pineapple roller Thelenota ananas (susuhan) and the red sea cucumber Holothuria edulis.

Other uses for the starfish they recover include the manufacture of animal feed, composting and the preparation of dried specimens for the arts and craft trade.