The spermatozoa of Hirudinea with examples from three different taxa

The ultrastructure of mature spermatozoa from three leech species, Haemadipsa zeylanica (. Moquin-Tandon, 1826) (Haemadipsidae), Theromyzon tessulatum (. Müller, 1774) and Placobdella costata (Fr. Müller, 1846) (Glossiphoniidae), was investigated by means of transmission electron microscopy. While the three species showed the general features of hirudinid spermatozoa, considerable variation was found in nearly all characters examined: the anterior acrosome varied in length and in shape the nuclei also varied in size and show a very complicated shape which is maintained for the whole length in the two glossiphonids, while in the haemadipsid, it showed two different morphological regions. The single mitochondrion, which is surrounded by an electron-dense sheath, is straight in H and T. tessulatum, but twisted in P. costata. The flagellum in the three studied species is of the prominent central sheath type. The basal body is absent in the three species and the central apparatus of the flagellum was observed penetrating to the miochondrion. By comparing the present data with those from "oligochaeta", Branchiobdellida and Acanthobdellida, we found that only the presence of an anterior acrosome characterizes the true leeches and at present should be regarded as an autapomorphic character of Euhirudinea.