To the group of echinoderms belong the seaurchins (echinoids), sealilies (crinoids), starfish, brittle snakes and sea-cucumbers. Apart from a few primitive ancestors, all these animals have pentamerous (fivefold) symmetry: starfish mostly have five arms and sea lilies have five arms or a multiple of five. The body plan of echinoids and sea cucumbers too is reducible to a pentamerous symmetry. Another important feature of this group is the possession of a water-vascular system: an ingenious system of vessels through which sea water flows and which supplies the pressure differential for their suckered feet to grip with.

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