Histological, histochemical, biochemical, and ultrastructural studies show that digestion inSyndesmis franciscana(Lehman, 1946), a rhabdocoel entocommensal in the coelom and intestine of the sea-urchinStrongylocentrotus purpuratusfrom California, is both extra- and intracellular, involving an endopeptidase, exopeptidases, phosphatases, and a lipase. The gastrodermal cells are loaded with lipid globules, so that this substance forms over 25% of dry weight. Glycogen accounts for a further 19% dry weight. Over 85% of the glycogen is in the solublelyo-form, as in adult endoparasitic helminths. Both the form and total amount of glycogen present suggest that the pattern of carbohydrate metabolism is more akin to the parasitic than the free-living platyhelminths.Syndesmis antillarumStunkard and Corliss, 1951, a syndesmid from the Caribbean, is shown to differ both morphologically and physiologically fromS.franciscana. The present synonymy of these two species is considered unjustified and incorrect.