a free-swimming larva of sea urchins. Echinoplutei have three to four pairs of long “arms,” a gut, and three pairs of coelomic saccules. The margins of the perioral depressions and the surface of the “arms” bear long cilia, by means of which the larva swims. Several months are required to complete the development of an echinopluteus. The body of a young sea urchin forms on the left side of the larva from a section of the ectoderm, two coelomic saccules, and the midgut. The remaining larger part of the echinopluteus’ body dies off in the process of its development.