ULTRASTRUCTURE OF TETRAONCHOIDES (PLATYHELMINTHES, MONOGENEA) - TEGUMENT, TEGUMENTARY RECEPTORS, OOCYTES AND MINERALOUS CORPUSCLES

Tetraonchoides sp. (Platyhelminthes, Monogenea, Monopisthocotylea, Tet raonchoididae), a gill parasite of the fish Uranoscopus sp. in Dakar, Senegal, was studied by transmission electron microscopy. In most of i ts surface, the external syncytial layer of the tegument has one of th e simplest structures found in the Monogenea. The syncytium is thin (0 .75 mu m) with apical and basal membranes almost rectilinear, very few cytoplasmic inclusions, and almost no microvilli. Tegumentary recepto rs are of the uniciliary type. The tegumentary syncytium bears microvi lli which form a crown around the receptor, a structure previously und escribed. The bladder contains mineralous spherites, or calcareous cor puscles, 0.5-1.25 mu m in diameter, which are previously undescribed i n the Monogenea. Oocytes show cortical membrane-bound granules, 0.25 m u m in diameter. A comparison of data from the literature suggest that small (0.25-0.5 mu m) cortical granules could be considered a plesiom orphic character of the monopisthocotylean Monogenea and Digenea, wher eas polyopisthocotylean Monogenea have larger (0.5-1 mu m) granules, w hich may be considered apomorphic.