Polycercus lumbrici, previously described only by Metchnikov (1868) in Russia, has been found in Scotland in the earthworm Allolobophora terrestris.Experimental infestation of earthworms with Polycercus lumbrici as a result of feeding them with the eggs of Paricterotaenia paradoxa shows that the proliferating cysts are the larvae of the latter as suggested by Joyeux and Baer (1939).There is evidence that Polycercus may induce parasitic castration of the earthworm in which it occurs.Metchnikov's description of the development of the larva is confirmed for the most part but is modified to include a retroversion of the larva in its early stages of development. The development of the scolex is shown to follow a pattern similar to that of Cysticercus fasciolaris in that the hooks are borne on a ‘prebulb’ which later invests a ‘bulb’, the former giving rise to the muscles of the walls of the double sac of the rostellum, the latter to the hook-elevator muscles.The rostellar hooks of P. paradoxa are elevated by the action of a double layer of muscles which radiate from a conical invagination of the bulb. These muscles, in association with the muscles of the wall of the rostellum, elevate the hooks by modifying the shape of the rostellum rather than by direct action on the hooks.The nervous system and the excretory system in the scolex of P. paradoxa are briefly described and the nature of the ‘glandular’ structure of the rostellum is discussed.I wish to record my gratitude to Professor H. G. Callan for affording me the facilities for my work and to the many persons, too numerous to mention by name, who helped me to obtain my material. I am particularly indebted to Mr D. R. R. Burt for guidance and constructive criticism at all stages of my work and to Mr I. M. Sandeman for much helpful advice.The work was carried out during the tenure of Research Grants from Madras College, St Andrews, and from the D.S.I.R., to whom I would express my appreciation for their aid.