Spermiogenesis in the amphilinidean cestode Amphilina foliacea (Rudolphi, 1819) was examined using transmission electron microscopy. The orthogonal development of the two flagella is followed by a flagellar rotation and their proximodistal fusion with the median cytoplasmic process. This process is accompanied by extension of both the mitochondrion and nucleus into the median cytoplasmic process. The two pairs of electron-dense attachment zones mark the lines where the proximodistal fusion of the median cytoplasmic process with the two flagella takes place. The intercentriolar body, previously undetermined in A. foliacea, is composed of three electron-dense and two electron-lucent plates. Also new for this species is the finding of electron-dense material in the apical region of the differentiation zone at the early stage of spermiogenesis, and the fact that two arching membranes appear at the base of the differentiation zone only when the two flagella rotate towards the median cytoplasmic process. The present data add more evidence for a close relationship between the Amphilinidea and the Eucestoda.
The ultrastructure and histochemistry of the tegument and penetration glands of adult Amphilina foliacea from the body cavity and the tissues of the internal organs of Acipenser ruthenus and A. stellatus were studied. New data on the localization in the tissue, development and in encapsulation of the adult A. foliacea mostly in the liver of A. ruthenus were obtained. The well developed penetration glands are necessary for penetration into the tissue and for migration of A. foliacea into the body cavity of the hosts. The tegument of the adult A. foliacea is a syncytium with cytons deeply embedded into the parenchyma. The secretory activity of the tegument of worms has a protective function against the immune system of the host. Our results give further information about the phylogeny of Amphilinidea and confirm the view of the close phylogenetic relationship of Amphilinidea and Cestoidea.