Spermatozoon cytoarchitecture of Amphilina foliacea (Platyhelminthes, Amphilinidea).

The mature spermatozoon of Amphilina foliacea Rudolphi, 1819 has been examined using transmission electron microscopy. The male gamete is filiform and tapered at both extremities. Its moderately electron-dense cytoplasm possesses two parallel axonemes of unequal lengths with the 9 + "1" trepaxonematan pattern, a mitochondrion, a nucleus, parallel cortical microtubules, four electron-dense attachment zones, and electron-dense glycogen granules. A crested body is absent. The anterior extremity of the cell exhibits a single axoneme. The anteriormost cortical microtubules have been observed with the appearance of the second axoneme. The number of cortical microtubules reaches a maximum (up to 25) in the nucleated region III of the spermatozoon. A single mitochondrion extends from the middle of region II to the end of region III of the cell. Both axonemes have become disorganized in a similar way: the axonemal doublets disappear first, followed by the central core. The nucleus is surrounded by a few cortical microtubules in the proximal part of region V. In the distal extremity of the mature spermatozoon, there is only the nucleus. Differences of spermatozoon ultrastructure within Amphilinidea and other Neodermata are discussed.