Reproductive organs and spermatogenesis of the peculiar spermatozoa of the genus Kryptodasys (Gastrotricha, Macrodasyida), with an appraisal of the occurrence and origin of the tail‐less spermatozoa in Gastrotricha

The presence of aflagellate spermatozoa in two species of Kryptodasys (Fam. Macrodasyidae), a gastrotrich genus recently described, is reported. Their ultrastructure shows common features, which appear peculiar to the genus and different from those of male gametes of the phylogenetically unrelated Dolichodasys (Fam. Cephalodasyidae), the only other macrodasyidan genus known to have aflagellate sperm. Additional information on the ultrastructure of the genital apparatus of the two Kryptodasys species is given, which confirm the data of the originals description and the systematic delimitation of this new genus from Macrodasys, which was mainly based on the structure of the reproductive system until now. The functional and phylogenetic significance of the aflagellate condition of spermatozoa in the two taxa under study and in other species is discussed also in the light of the generally flagellate condition of functional sperm in Gastrotricha.