Gastrotricha Macrodasyida, aquatic worms, are primarily hermaphrodites with internal
cross-fertilization and indirect sperm transfer. Insightful information on this matter dates to 1978
and derives from an illuminating work focused on two Macrodasys species. The role as a “model”
of this taxon clashes, however, with the relative paucity of data concerning the ultrastructure of the
accessory organs involved in the sperm transfer. Therefore, we have undertaken a new study on
three additional species of Macrodasys focusing on the frontal and caudal organs and also on the
spermatozoa. Our observations confirmed the structural plan of the reproductive system reported
for the genus, but found some differences concerning the accessory reproductive organs. Most
importantly, we found spermatozoa inside the caudal organ of several single specimens, suggestive
of a more plausible hypothesis about sperm transfer modality in Macrodasys. In the spermatozoa, the
position of the mitochondrion outside rather than inside the nucleus has been confirmed as a major
difference between the spermatozoa of Macrodasys species and those of the other Macrodasyida. We
suggest that the peculiar structure of the mature spermatozoon of Macrodasys could derive from an incomplete process of spermatogenesis and hence it would be useful to infer phylogenetic alliances
