BY
Abstract.--The 54 putative species of the sipunculan subgenus Phascolosoma (Phascolosoma) and the morphological characters used to differentiate them are critically reviewed. The monograph of Stephen & Edmonds (1972) is used as a starting place and all changes made in the intervening years are reiterated here. Available type material was studied and new collections of Hawaiian and Caribbean worms are used to analyze within-deme variation. Twenty chromosomes of gradually differing sizes are present. Four characters are broadly useful at the species level (number of hook rings, secondary tooth, pigment on introvert, and pre-anal papillae). Eight characters can be used in a more restricted manner for subsets or special cases (hook size, tip angle, internal clear areas, basal elaborations, plus nephridia length, trunk papillae shape, papillae platelets, and retractor origins). Six are of no value to the taxonomist (hook presence, introvert length, number of longitudinal muscle bands, nephridiopore/anus relationship, rectal caecum, and contractile vessel). A key to, and a discussion of, the 16 remaining species (plus two reduced to subspecies) with the newly designated synonyms are presented. A brief statement of the distribution of each species is given. An overall summary of the zoogeography and habitat shows the border area between the Indian and Pacific Oceans to be the most diverse (81%) while only 19% (three species) live in the Caribbean. With a few exceptions this genus lives in warm, shallow waters and hard substrates.