Echinodermata are animals with calcareous plates forming a more or less rigid skeleton, or with scattered plates and spicules embedded in the body wall. Many are provided with spines. All are marine, and all but a few sea cucumbers are benthic.
Class Holothuroidea. The sea cucumbers are mainly benthic. only members of the order Pelagiothurida being planktonic. There are over 650 species, some living in abyssal regions.
Class Asteroidea. The sea stars are among the most conspicuous of shore animals, but they live also at very great depths. About 1100 species are known.― 312 ―
Class Ophuroidea. There are more than 1600 species of brittle stars, with a wide horizontal and bathymetric distribution.
Class Echinoidea. There are about 600 species of sea urchins and sand dollars, a few of which live in deep water.
Class Crinoidea. About 800 species of sea lilies and sea feathers are known, with the center of distribution in the East Indian waters, but they also occur in many other waters. The former live mainly in the deep sea and are anchored by long stalks. The latter occur mainly at shallower depths and are without stalks. The class is a vanishing remnant of a formerly abundant group that has left more than 2000 fossil species.