☞ The species usually have an exterior calcareous skeleton, or shell, made of many pieces, and often covered with spines, to which the name. They may be star-shaped, cylindrical, disk-shaped, or more or less spherical. The body consists of several similar parts (spheromeres) repeated symmetrically around a central axis, at one end of which the mouth is situated. They generally have suckers for locomotion. The group includes the following classes: Crinoidea, Asterioidea, Ophiuroidea, Echinoidea, and Holothurioidea. See these words in the Vocabulary, and also

Ambulacrum

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