ULTRASTRUCTURE OF SPERMIOGENESIS AND SPERMATOZOA IN 3 ACOELS (PLATYHELMINTHES)

The spermatozoa of three species of marine acoels, Actinoposthia bekle mischevi (Childiidae) and Aphanostoma virescens (Convolutidae) from th e White Sea, and Baltalimania agile (Convolutidae) from the Barents Se a were studied by transmission electron microscopy. In Act. beklemisch evi, flagella fuse with the sperm body during spermiogenesis, and the nucleus becomes flattened, elongated, and uniformely electron dense in mature spermatozoa. In the three species studied, the spermatozoa hav e two incorporated axonemes. In Aph. virescens and B. agile axonemes h ave a 9 + 2 pattern with 9 + 0 distal extremities ; the notation 9 + 2 /9 + 0 is proposed for these axonemes. A study of the literature shows that this 9 + 2/9 + 0 pattern exists in other Acoels, particularly in the Convolutidae and Sagittiferidae, and that in the Acoela, axonemes with a true trepaxonematan 9 + ''1'' pattern (i.e. with a central spi ral) probably do not exist, although 9 + ''1'' axonemes with a differe nt central structure exist in some species. Two distinct origins are h ypothesized for the longitudinal microtubules arranged in lines or cir cles in the center of acoel spermatozoa : a) peripheral microtubules w hich migrate toward the center of the cell along a longitudinal groove and b) hook-like distal extremities of 9 + 0 axonemes made up of 9 si nglets and bent anteriorly in the sperm body.