Sea cucumbers belong to the Echinodermata even as sea-urchins or britle stars. Most living echinoderms are pentameral; they have fivefold symmetry, with rays or arms in fives or multiples of five. All echinoderms are marine; none can live in fresh water or on land. Sea cucumbers are common in oceans all over the world; in fact, in certain parts of the deep sea, the ocean floor swarms with vast herds of Sea cucumbers. They are also common in shallow-water habitats such as tidepools. While most are benthic, a few are pelagic.
Sea cucumbers are generally long and wormlike; they don't look much like starfish or sea urchins. However, they retain pentameral symmetry as all Echinodermata, with five rows of tube feet running from the mouth along the body. Sea cucumbers retain the skeleton of echinoderms, but in most species the skeletal plates are reduced to microscopic spicules, often shaped like wheels, bars, or anchors.
Sea cucumbers have an unusual method of respiration: they take in water through their anus to breathe. When disturbed or frightened, some sea cucumbers pour out a mass of sticky white threads to confuse or trap their enemies. Others are capable of releasing toxins which in aquaria have been known to kill all the animals and including the sea cucumbers themselves.
