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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.

Mitotic and meiotic chromosomes of Caryophyllaeus laticeps (Pallas) from Rybinsk reservoir were studied using air-drying techniques. It was established that the diploid set contains 20 metacentric chromosomes. The chromosomes are large--their mean absolute length ranges from 3.00 to 12.90 microns. The last pair of small homologues comprises only 3.4% of the total length of the complement. One specimen of C. laticeps with 2n = 21 (trisomic after the last pair of chromosomes) and one triploid specimen with 3n = 31 is described. Data on the comparative karyology of caryophyllids are discussed.