Fertilization and the functional anatomy of the germarium in the monogenean Entobdella soleae

The germarium of the monogenean skin parasite Entobdella soleae encloses a coiled tube containing many mature oocytes. This intragermarial tube communicates distally with the oviduct. The proximal end of the tube is closed by a "plug cell" which is suspended by finger-like cytoplasmic processes attached to the sheaths enclosing dorso-ventral muscle fibres passing through the main compartment of the germarium. Mature oocytes reach the tube lumen by penetrating the plug cell cytoplasm and this may be achieved by amoeboid movement of the oocyte. The tube contains spermatozoa and fertilization begins soon after a mature oocyte enters the tube. The events of fertilization have been determined by sequential ultrastructural studies of oocytes in the tube.