A new species of the holopodid genusCyathidiumwas found on rocks off Grande Comore in a depth of around 200 m. Based on external morphology of resting animals, the new speciesCyathidiumplantei sp. n is described, with emphasis on comparison to the only other extant species (C. foresti) as well as to the four extinct representatives of the genus. Concerning morphological characters, the new species is almost identical to the CretaceousC. depressum. A cladistic analysis of the entire family, including the genus Holopus, shares a peculiar pattern of bending of the arms, which in principle is an apomorphic character of the family and in detail shows variations within the family. In addition, stratigraphic data are used for the determination of the evolutionary direction. This analysis reveals that the two recent species are closely related to each other, and to the fossilC. depressum. from which the entire family is probably derived.