ECHINODEN'MATA (Gr. spiny-skinned), a class of radiate animals, the highest in organization of that great division of the animal kingdom. They have a digestive and a vascular system: fol. the former, however, there is in many of them only a single ori fice: a circular and radiating nervous system has been observed in many; they are espe cially characterized by their well-organized skin, which in many is strengthened by calcareous plates, and in some also has the additional protection of numerous long spines. Echinida3 (sea-urchins) exhibit these characteristics in greatest perfection. A ate riacice (star-fishes), ophinridee (brittle-stars), crinoidea, holothurider (sea-slugs, sea-cucum bers, etc.), and sipunculida, are also ranked among the E., and have been variously arranged in orders by different naturalists. Spines arc wanting in most of them; in some (holothurida and gipunculida), there are not even calcareous plates, and there is no incon siderable departure from the ordinary and perfect radiate form, an approach being made to the forms of mollusks and worms, whilst yet the accordance with the other E. is very
perfect in other parts of the organization. Almost all the E. are free, moving about at the bottom of the sea; some of them—at-least in an immature state—are stalked and fixed. They arc provided with "an apparatus for Water eilonlation," peculiar eliarac teristic of radiate animals, and which "can scarcely be said to exist in any of the other types." By means of this it is that they fill and fit for use the suckers or ambulaera (q.v.) with which most of them are provided, but of which the sipuneulidos are destitute. The spines as well as the ambulacra of the E. are used by those which possess them (echinida and ophiuridce) as organs of locomotion. —The British E. are described by Dr. Edward Forbes in an interesting work, entitled A History of British Starfishes and other Animals of the Class Eehinodermata (Loud. 1841).