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They have a more or less elongated body, often flattened beneath, and a circle of tentacles, which are usually much branched, surrounding the mouth; the skin is more or less flexible, and usually contains calcareous plates of various characteristic forms, sometimes becoming large and scalelike. Most of the species have five bands (ambulacra) of sucker-bearing feet along the sides; in others these are lacking. In one group (Pneumonophora) two branching internal gills are developed; in another (Apneumona) these are wanting. Called also , , and .