An electron microscopic study was performed to observe the ultrastructure of the tegument of F. seoulensis. The outer surface of the tegument was covered with a trilaminated plasma membrane. The electron-dense cytoplasmic layer was 2.5 microns wide in the anterior portion and contained numerous vacuoles, mitochondria and granular materials in its matrix. The basement layer was 330 nm wide or so, and its numerous extensions protruded into the cytoplasmic layer. The sensory organ was composed of a small vesicle of 1.7 x 1.1 microns in dimensions, which possessed a cilium of 1.2 x 0.19 micron in size. The pharynx was composed of the epithelial layer of about 0.5 micron wide, well developed muscle layer and basement layer. The tegument of the oral sucker was composed of a cytoplasmic layer of 0.4-0.5 micron width, a narrow basement layer, a well developed muscle layer and tegumental cells. Some kinds of secretory granules that seemed to be originated from the cells of the oral sucker were observed in the parenchymal portions of the adjacent cells. The tribocytic organ consisted of numerous microvilli. The microvilli were 5 nm wide and heptalaminated. Two types of secretory granules originated from the gland cells of tribocytic organ were observed in the tegument and parenchyme. The tegumental cells were irregular in shape, and of which nuclei were multifarious.
