Abstract

This is the first reported study of the ultrastructure of sperm and spermatogenesis of a terrestrial triclad (Terricola). Processes from the cells of the testis wall envelop early germinal cells, and wall cells are ciliated in the transition zone to the sperm duct. Following meiosis of spermatocytes, spermatids develop by formation of a zone of differentiation containing peripheral microtubules, two centrioles facing in opposite directions, a prominent intercentriolar body and a semicircle of dense material around the centrioles. Centrioles become basal bodies giving rise to free flagella. Elongation of this zone, accompanied by rotation of the basal bodies around the shaft, results in flagella being carried to a distal and subterminal location, emerging on the one side of, and perpendicular to, the spermatid shaft. Nucleus and mitochondrion migrate into the shaft. Artioposthia is compared with other triclads and other turbellarian groups. The lack of dense bodies in sperm and of rootlets during spermiogenesis may be autapomorphies for the Tricladida. To our knowledge, the only report of splitting of flagellar tips into a number of separate threads, outside Tricladida, is from Syndisyrinx punicea (Rhabdocoela: Umagillidae) (Rohde and Watson 1988).