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Summary
Mature spermatozoa of Nematogenia panamensis (Annelida: Oligochaeta, Ocnerodrilidae) from Madagascar were studied by electron microscopy. The spermatozoon is 35 μm long and shows the conventional clitellate sequence of acrosome, nucleus, middle piece and tail. The acrosome is asymmetric, showing an acrosome rod consistently bent to one side and probably exiting laterally for a short distance from the acrosome vesicle, only to re-enter apically. The middle piece consists of six mitochondria with the shape of a cylindrical sector and the tail is a flagellum with a 9+2 axoneme with two central tetragon fibers, surrounded for most of its length by glycogen granules. While the general features of Nematogenia spermatozoon are undoubtedly of megadrile type, some characters, like the shortness of the acrosome and the basal chamber, indicate a plesiomorphic condition within the group. This is in good agreement with the proposed phylogenetic position of Ocnerodrilidae within the Oligochaeta.