The wall of the gonad ofGoniodiscus rotundatus, according to ultrastructural observations, may be subdivided in functional unities similar to the acina ofLimnaea stagnalis. Every U shaped functional unity has cuboidal cells (cells 1) at the proximal ends, and flattened cells (cells 2) at the bottom. The transitional zone between the two cells types is just the same as the “germinal epithelial ring”. Ultrastructural study confirms the subdivision of the gonad in a series of periferically located female compartments and a central male one, which are separated by “somatic” cells.