Spermiogenesis and ultrastructure of mature spermatozoon of the caryophyllidean cestode Hunterella nodulosa, a parasite of suckers (Catostomidae), have been studied by transmission electron microscopy. This monozoic tapeworm is unique in its mode of attachment and represents the second North American species studied. The process of spermiogenesis of H. nodulosa follows the general pattern already described in other caryophyllideans. The most characteristic feature is the presence of a slight rotation of the flagellar bud, which seems to be a typical character of spermiogenesis in this cestode group. The mature spermatozoon of H. nodulosa is characterized by the presence of one axoneme of 9 + ''1'' type of the trepaxonematan flatworms surrounded by a semi-arc of cortical microtubules in its anterior extremity, parallel nucleus and cortical microtubules arranged in a parallel pattern, which corresponds to the Type III pattern of cestode spermatozoa according to Levron et al. (2010). Comparison of the present data with those available for other caryophyllideans did not reveal substantial differences, even though they belong to different families, infect different hosts (catostomid, cyprinid and siluriform fishes) and occur in distant zoogeographical regions. This indicates uniformity of the process of sperm formation and spermatozoon ultrastructure in one of the evolutionarily most ancient groups of tapeworms.
The scolex surface of the mature spathebothriidean Cyathocephalus truncatus (Pallas, 1781), a parasite of the brown trout Salmo trutta fario L., was studied using scanning and transmission electron microscopy. A particular attention was paid to microtriches, unique structure on the surface of the Cestoda. The scolex of C. truncatus is covered with two types of filiform microtriches (filitriches): aciculate (≈ 3 µm long) and capillate (≈ 10 µm long). Capillate microtriches, which have never been reported in any other spathebothriideans, are described for the first time using transmission electron microscopy. The tegument covered with filiform microtriches only (no spiniform microtriches are present) is typical of cestode groups supposed to be the most basal, e.g., Gyrocotylidea, Spathebothriidea, and Caryophyllidea.
The surface structures and gland cells of the posterior rosette organ of Gyrocotyle urna Grube et Wagener, 1852, a member of the group presumed to be the most basal of the tapeworms (Cestoda: Gyrocotylidea), was studied by scanning electron and transmission electron microscopy. Surface structures on the outer (oriented away from the intestinal wall) and inner (in contact with the intestinal wall) rosette surfaces differ from each other and represent a transitional form between microvilli and microtriches typical of tapeworms (Eucestoda). The inner surface of the rosette possesses numerous glands. On the basis of the size and electron-density of their secretory granules, three types of unicellular gland cells can be distinguished. The least common type (Type I) is characterized by the production of small, round, electron-dense granules of about 0.3 µm in diameter, whereas another type of secretion (Type II) is formed from homogenous, moderately electron-dense, spheroidal granules of about 0.7 µm in diameter. The most common type of glands (Type III) is recognized by a secretion comprising large, elongate, electron-dense granules of about 1 µm long and 0.5 µm broad. The secretory granules of the three types of the glands are liberated by an eccrine mechanism and the gland ducts open via small pores on the inner rosette surface. The complex of secretory glands of the posterior rosette of G. urna is similar to those in the anterior attachment glands of monogeneans (as opposed to the types of glands present in other helminth groups). However, the tegumental surface structures of Gyrocotyle are supporting evidence for the relationship between the Gyrocotylidea and Eucestoda.
The vitellogenesis of Paraechinophallus japonicus (Yamaguti, 1934), the first pseudophyllidean tapeworm of the family Echinophallidae studied using transmission electron microscope, is described on the basis of ultrastructural observations of specimens from the benthopelagic fish Psenopsis anomala (Temminck et Schlegel, 1844) (Perciformes: Centrolophidae). The process of vitellogenesis in P. japonicus follows the same general pattern observed in other tapeworms. Five stages of vitellocyte development have been distinguished. The first stage corresponds to immature cells containing ribosomes and mitochondria. The second stage of development is characterized by the appearance of granular endoplasmic reticulum and Golgi complexes, formation of shell globules and lipid droplets at the periphery of the cell cytoplasm. Vitellocyte of the third stage presents accumulation of shell globules and lipid droplets. During the fourth stage, shell globule clusters are formed, and lipid droplets and rosettes of α-glycogen are accumulated. Mature vitelline cells are characterized by a great number of lipid droplets with glycogen in the centre of the cytoplasm, whereas shell globule clusters are situated more peripherally. The interstitial tissue of vitelline follicles of P. japonicus is syncytial with long cytoplasmic projections extending between vitelline cells. The presence of a large amount of lipid droplets in the vitelline cytoplasm within the eggs of P. japonicus may be related to egg accumulation in the uterine sac.