There is little information available about the internal anatomy of the polychaete family Cossuridae Day, 1963. In the interests of clarifying the systematic position of this group, we describe the structure of the abdominal body wall, coelomic organization, spermatogenesis and oogenesis ofCossuracf.longocirrataWebster and Benedict, 1887. There is a spacious coelomic cavity in the abdominal region unlike the thoracic region of the body, though as in the thorax there is no outer peritoneal lining. Muscle layers are virtually absent in certain areas of the abdomen, explaining the tendency for cossurids to disintegrate very easily. Oogenesis in this species is extraovarian but each oocyte is ensheathed by a few follicle cells. The follicle cells appear to be of the “inactive” type since they show no microvilli of the outer surfaces, and no rER or Golgi bodies. Spermiogenesis occurs with synchronously developing clusters of cells attached to a central cytophore. Mature sperm have a spherical head, a conical acrosome and a simple midpiece. This morphology is typically seen in species using external fertilization and thus the sperm are probably classifiable as ect-aquasperm, though this should be confirmed. The present results do little to help with the systematic placement of the Cossuridae. Further detailed study of gametogenesis and other organ systems across the Polychaeta are required in this regard.