The tapeworm Archigetes sieboldi Leuckart, 1878 (Platyhelminthes: Cestoda: Caryophyllidea) has been cited as a likely representative of the "protocestode" condition, owing to its lack of segmentation and ability to attain sexual maturity in the invertebrate host (aquatic oligochaetes). The idea has been variously amplified or rejected in the literature, although the actual phylogenetic position of the species has not been investigated until now. New collections of Archigetes sp. from both its vertebrate and invertebrate hosts provided the opportunity to estimate its phylogenetic position with the use of molecular systematics, while prompting new analyses aimed at assessing the early diversification of the Cestoda. Additional collections representing the Amphilinidea, Caryophyllidea, and Gyrocotylidea were combined with published gene sequences to construct data sets of complete 18S (110 taxa) and partial (D1-D3) 28S (107 taxa) rDNA sequences, including 8 neodermatan outgroup taxa. Estimates resulting from Bayesian inference, maximum likelihood, and maximum parsimony analyses of the separate and combined data sets supported a derived position of the genus within the Caryophyllidea, and thus reject the idea that Archigetes sp. may exemplify a "primitive" condition. Topological constraint analyses rejected the hypothesis that Archigetes represents the most basal lineage of the Eucestoda, but did not rule out that it could represent the earliest branching taxon of the Caryophyllidea. In all analyses, the Eucestoda were monophyletic and supported basal positions of the nonsegmented Caryophyllidea and Spathebothriidea relative to other major lineages of the Eucestoda, implying that segmentation is a derived feature of the common ancestor of the di- and tetrafossate eucestodes. However, constraint analyses could not provide unequivocal evidence as to the precise branching patterns of the cestodarian, spathebothriidean, and caryophyllidean lineages. Phylogenetic analyses favor the interpretation that sexual maturity of Archigetes sp. in the invertebrate host, and similar examples in members of the Spathebothriidea, are the result of progenesis and have little if any bearing on understanding the protocestode condition.

Spermiogenesis and the ultrastructural features of the spermatozoon of Diphyllobothrium latum (Cestoda, Pseudophyllidea) are described using transmission electron microscopy. Spermiogenesis is characterized by the development of two flagella of unequal length that grow asynchronously. When the first growing flagellum starts to rotate, the second one develops. Flagellar rotation is thus asymmetric and asynchronic. It is followed by proximodistal fusion with the median cytoplasmic process. Electron-dense material is present in the apical region of the zone of differentiation in the early stages of spermiogenesis. The intercentriolar body consists of seven plates: three are electron-dense. Four attachment zones occur in the median cytoplasmic process. An atypical arrangement of striated roots was occasionally observed. The mature spermatozoon possesses two axonemes of 9 + "1" trepaxonematan pattern, nucleus, cortical microtubules, electron-dense granules, and lacks mitochondria. The ultrastructure of the anterior extremity of the spermatozoon in D. latum clearly differs from that in the bothriocephalid pseudophyllideans, mainly in the absence of a crested body and a ring of electron-dense tubular structures. The spermatological data support the assumption that the order Pseudophyllidea is formed by two unrelated clades, "Bothriocephalidea" and "Diphyllobothriidea."

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In the spathebothriidean tapeworm Didymobothrium rudolphii (Monticelli, 1890) the fine structure of the vitellocytes at different stages of their development within the vitelline follicles, vitelline ducts and uterus was studied for the first time using transmission electron microscopy. The vitellocyte inclusions of D. rudolphii are shell globule clusters containing tightly packed shell globules associated with a matrix of moderate electron density, glycogen granules, large electron-lucent lipid droplets (up to 3 μm in diameter), and, occasionally, a lipid droplet may occur in the nucleus of the vitellocytes. The diameter of the clusters ranges from 0.4 to 2.5 μm, the number of shell globules in the clusters varies from 8 to 45, and the size of the globules ranges from 0.12 to 0.25 μm and they are of approximately homogeneous sizes within a cluster. Most vitellocyte lipid droplets have a heterogeneous configuration with a ‘cavity’ inside them when they are within vitelline ducts and intrauterine eggs. Vitellocytes of the eggs contain dark concentric bodies and lipid droplets. The interstitial tissue has a syncytial structure. The morphological parameters of the diameter and shape of shell globule clusters, arrangement of shell globules in clusters, number and diameter of globules within clusters, types of lipid droplets and presence of dark concentric bodies are compared with those of two other spathebothriidean genera, Cyathocephalus and Diplocotyle. The comparative data demonstrate that vitelline material morphology has unique features in three spathenothriidean genera and may be used as evidence for the recognition of separate taxa.

Abstract. Ultrastructural characteristics of progenetic and monoxenic Archigetes sieboldi Leuckart, 1878 from the oligochaete Limnodrilus hoffmeisteri Claparède are described. Our observations demonstrate that progenetic Archigetes sieboldi shares characteristics of both larval (progenetic) and adult stages. The primary larval characteristics are: the presence of a cercomer; a surface filamentous coat covering the whole worm; the presence of the penetration glands and the absence of tegumental ones; wide sarcoplasmic processes connecting the circular and longitudinal external tegumental muscles; the absence of the dense homogenous zone of the basal lamina beneath the epithelial cytoplasm of all reproductive organs and ducts; non-functional gonopores; and an orthogonal plan of nervous system with three pairs of longitudinal nerve trunks. The principle adult characteristics are: oogenesis, spermiogenesis and vitellogenesis that produce fertilized eggs; the uterine glands; a well-developed longitudinal tegumental muscle layer between tegumental cytons; and the presence of different microtriches. As a result of this progenetic development there has been a secondary reduction in the life cycle of A. sieboldi. It is postulated that a similar process of progenesis may have played a major role in the early evolution of the Caryophyllidea by first appearing in a plerocercoid stage of an ancestral strobilate cestode from fish.