A historical review of research, carried out on the phylum Acanthocephala, to the present time is given as a background to the research problem undertaken here. Detailed observations on the morphology of Octospinifer macilentus were made and the functional significance of various structures encountered in the praesoma are discussed. The histology and cytology of the parasite was investigated by a variety of staining techniques and by means of the light and electron microscopes. This thesis contains the first ultrastructural report on the Order Eoacanthocephala. The ultrastructure of the lemnisci, embryonic envelopes of the acanthor, tegument and the tegumentary musculature is described in detail. The functional significance of the various observations are discussed together with pertinent literature of similar research on other species of Acanthocephala.