1. Two species of the dicystid gregarine Polyrhabdina Mingazzini have been studied in polychaete worms from Plymouth. (a) Polyrhabdina spionis is not the organism described by Kölliker as Gregarinaspionis, and should be named Polyrhabdina spionis Mingazzini. (b) We do not consider that the epimerite of Polyrhabdina spionis is intracellular, nor do we believe that it is an amoeboid and food-absorbing structure. The specimens we have found in Scolelepis fuliginosa differ in other details from those described by Caullery and Mesnil in the same worm from another locality, and may constitute a new variety. (c) The dicystid gregarine from the intestine of Polydora flava is described by us. This may be the organism called by Caullery and Mesnil Polyrhabdina polydorae, but we do not agree that this name is synonymous with Doliocystis polydorae Léger. 2. We describe two new dicystid gregarines from the intestines of the echiurid worm Thalassema neptuni Gärtner at Plymouth. These we have named Hentscheliathalassemae n.gen., n.sp., and Lecythion thalassemae n.gen., n.sp. 3. Association was frequently observed in the gregarines from Thalassema. The gametocytes do not normally develop further until they have been passed out with the excrement. In sea-water a cyst is formed and spores developed. Anisogamy, with flagellated male gametes, has been observed. 4. The dicystid gregarines are a heterogeneous group. Those with simple, transitory epimerite may be related to the acephalines; but the species dealt with in this paper are almost as complicated in structure as tricystids, and seem to differ from these only in the absence of a septum dividing a protomeritic from a deutomeritic segment.