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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. Echinoderms are important both ecologically and geologically.
Brittle stars are not used as food, [1] though they are not toxic, because of their strong skeleton. Even if some species have blunt spines, no brittlestar is known to be dangerous, nor venomous. There is no harm evidence towards humans, and even with their predators, brittlestars' only means of defense is escaping or discarding an arm.
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The ambulacral canals are used in their water vascular system which is another characteristic that binds this phylum together. The water vascular system develops from their middle coelom or hydrocoel. Echinoderms use this system for many things including movement by pushing water in and out of their podia or "tube feet".
These are composed of pigmented epithelial cells that respond to light and are covered by a thick, transparent cuticle that both protects the ocelli and acts to focus light. Many starfish also possess individual photoreceptor cells in other parts of their bodies and respond to light even when their eyespots are covered. Whether they advance or ...
F. A. Bather's diagram of Echinoderm phylogeny from A Treatise on Zoology, Part III: The Echinoderma" (1900). Originally defined by F. J. Bell in a sense that excluded Holothuroidea, Eleutherozoa was expanded by F. A. Bather in his 1900 taxonomy to include all free-living echinoderms.
These apparently produce coelomocytes, amoeboid cells somewhat similar to the blood cells of vertebrates. [11] Although the contents of the water vascular system are essentially sea water, apart from coelomocytes, the fluid also contains some protein and high levels of potassium salts. [11]: 933–939
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