In Parvulastra parvivipara, an intragonadal brooder, the young starfish obtain nutrients by eating other eggs and embryos in the brood pouch. When the prey is a clam or other bivalve, the starfish pulls with its tube feet to separate the two valves slightly, and inserts a small section of its stomach, which releases enzymes to digest the prey. The steroids are mostly saponins, known as asterosaponins, and their sulphated derivatives. The entrance to this is known as the blastopore and it will later develop into the anus—together with chordates, echinoderms are deuterostomes, meaning the second (deutero) invagination becomes the mouth (stome); members of all other phyla are protostomes, and their first invagination becomes the mouth. [91], The starfish are a large and diverse class with about 1,500 living species. [70] Starfish are vulnerable to high temperatures. [145], Starfish has repeatedly been chosen as a name in military history. [88] It is derived from the Greek aster, ἀστήρ (a star) and the Greek eidos, εἶδος (form, likeness, appearance). Square Biscuit Starfish is distinctively bright orange in color, which can grow up to 6 centimeters. At the base of each arm are paired gonads; a lateral vessel extends from the genital ring past the gonads to the tip of the arm. [29] Those starfish that brood their eggs by "sitting" on them usually assume a humped posture with their discs raised off the substrate. In particular, Oreaster reticulatus, with its easily accessed habitat and conspicuous coloration, is widely collected in the Caribbean. [39] In temperate regions, the spring and summer brings an increase in food supplies. [2] Living asteroids, the Neoasteroidea, are morphologically distinct from their forerunners in the Paleozoic. The vessels form three rings: one around the mouth (the hyponeural haemal ring), another around the digestive system (the gastric ring) and the third near the aboral surface (the genital ring). [13], When longitudinal muscles in the ampullae contract, valves in the lateral canals close and water is forced into the tube feet. [13] Gas exchange also takes place through other gills known as papulae, which are thin-walled bulges on the aboral surface of the disc and arms. They are found from the intertidal zone down to abyssal depths, 6,000 m (20,000 ft) below the surface. It casts off its stalk and becomes a free-living juvenile starfish about 1 mm (0.04 in) in diameter. [37][38], In the tropics, a plentiful supply of phytoplankton is continuously available for starfish larvae to feed on. [82], Echinoderms first appeared in the fossil record in the Cambrian. [14] Other chemicals and relaxation of the ampullae allow for release from the substrate. [11] The structures are supported by collagen fibres set at right angles to each other and arranged in a three-dimensional web with the ossicles and papulae in the interstices. Fluid movement is bidirectional and initiated by cilia. [2] The phylogeny proposed by Gale in 1987 is:[2][117], Valvatida, including Velatida, Spinulosida (not a clade)[2], The phylogeny proposed by Blake in 1987 is:[2][118], Later work making use of molecular evidence, with or without the use of morphological evidence, had by 2000 failed to resolve the argument.

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