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Echinoderms are marine life forms, and the group includes brittle_stars, sea_urchins, sea_cucumbers, sea_lilies, and sea_stars (starfish). These animals lack the ability to osmoregulate, so they are not found in brackish waters. Their phylum name is based upon the presence of sharp external spines on their body surface.

These animals have a calcium-based endoskeleton (internal skeleton). Echinoderm zygotes become bilaterally_symmetrical free-swimming_larva; but their bodies specialize towards radial_symmetry as they mature. Sea stars are pentaradial. No other group of animals this advanced expresses radial symmetry.

These organisms feed upon clams as well as other invertebrates. A single starfish may eat as many as fifteen clams per day. These animals have a complete_digestive_system, which includes a separate mouth and an anus. The mouth is located on the ventral surface and the anus is on the dorsal surface. The cardiac_stomach protrudes from the mouth to surround prey. Each of the five arms of the starfish contains numerous digestive_glands (technically, these are called pyloric_ceca).

Starfish have a water-vascular_system, which is a hydraulic system of fluid-filled canals that attach to the tiny tube_feet. The tube feet are located on the ventral side of the body, and can be used for motility or for attachment to prey. The canals of the water-vascular system provide the fluid pressure that operates the tube feet. The water vascular system includes the madreporite (openings on the dorsal surface - function still obscure, but may assist in pressure regulation during sudden changes in water pressure), stone_canal (connects the madreporite to the ring canal, which is located on the dorsal surface of the central disc), ring_canal (a circular canal in central_disc) and tube feet. The ambulacral_groove is attached to a series of plates that physically support the tube feet.

These animals lack excretory_organs, have a blood-vascular system, are coelomate and have separate sexes. The gonads of the starfish are located below the pyloric ceca in each of the five arms. Fertilization is external for most echinoderms.