The unique ultrastructure of the uterus of the Gyrocotylidea Poche, 1926 (Cestoda) and its phylogenetic implications.

The members of the order Gyrocotylidea are monozoic tapeworms and generally considered to be the most primitive group of the Cestoda in terms of the evolution of this platyhelminth class. As part of a series of ultrastructural studies on Gyrocotyle urna (Wagener, 1852), three regions of the uterus were distinguished. The proximal region of the uterus is characterised by underlying perikarya, the presence of septate junctions within the epithelial wall and two types of specialised outer coverings, lamellae and cilia. The middle, syncytial region of the uterus is covered by short lamellae and marked by a concentration of sunken glandular perikarya (uterine glands). Glandular spheroidal granules (c.0.45 microm in diameter) of moderately dense content and a fine fibrillar texture are liberated by migration through the luminal membrane. The epithelium of the sac-shaped, distal region of the uterine duct is interrupted by cytoplasmic processes of sunken epithelial bodies, covered with lamellae and contains septate junctions. A muscular sphincter surrounds the narrow, terminal region of the distal uterine duct. The ultrastructural pattern of the uterus of the Gyrocotylidea has important discriminating traits (i.e. the presence of sunken perikarya along its entire length, septate junctions within the uterine epithelial cytoplasm of the proximal and distal regions, and cilia on the surface of its proximal region) unique among the Neodermata and which may represent autapomorphic character states for the group.