The sea cucumber does not look like any other member of the echinoderm family. Wormlike, with the mouth at one end of the horizontal body and the anus at the other, holothuroidea lack arms entirely. Instead, they have a row of tentacles around the mouth--from eight to 30 depending on the species. Every other member of the echinoderm family has arms.
The echinodermata is a big family, the largest marine family; that is, not one member lives in fresh water or on land, all dwell in salt-water. Most echinoderms have hard bodies with well-developed circular muscles, like the sea anemone and the sand dollar. The sea cucumber is the only member of the family to have a respiratory system.
The echinoderms are pentameral--that means that they have arms or rays in multiples of five. In this way, the sea cucumbers are like other members of the family. They have five rows of tube feet that run along their bodies. They also have the echinoderm's skeleton, although the skeletal plates are very tiny and in shapes of wheels, bars or anchors. According to the University of California Museum of Paleontology, "One way to think of a holothurian is as a sea urchin that is lying on its side, stretched out, and missing much of its skeleton! "
Holothuroidians are found only in sea water. In certain sections of the deep ocean, sea cucumbers completely cover the floor. But like other echinoderms, their habitat is not exclusively deep water. Tide-poolers often see holothurpoidians in the shallows as well.
The family of echinoderms includes sea-creatures with very diverse eating behavior. Some, like the holothurians and sand dollars, feed on detritus. Other, like the starfish, are predators. Still others, like sea urchins, scrape their food--mostly algae--from rocks.