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The phylogeny of the Rhabdocoela, a species-rich taxon of free-living flatworms, is reconstructed based on complete 18S rDNA sequences. The analysis includes 62 rhabdocoels and 102 representatives of all major flatworm taxa. In total, 46 new sequences are used, 41 of them from rhabdocoel species, five from proseriates. Phylogenetic analysis was performed using maximum parsimony and Bayesian inference. Clade support was evaluated with parsimony jackknifing, Bremer support indices and Bayesian posterior probabilities. The resulting cladogram corroborates that the Rhabdocoela is monophyletic, but its sister group remains uncertain. The ‘Dalyellioida’ and the ‘Typhloplanoida’, both former rhabdocoel subtaxa, are polyphyletic. Within the Rhabdocoela the monophyletic Kalyptorhynchia, characterized by a muscular proboscis, forms the sister group of all other rhabdocoels. The Schizorhynchia is a monophyletic subtaxon of the Kalyptorhynchia, with the split proboscis as a synapomorphy. Except for the Dalyelliidae and the Typhloplanidae, both freshwater taxa, none of the ‘families’ previously included in the ‘Typhloplanoida’ and the ‘Dalyellioida’ appears to be monophyletic. As a result of this analysis, three existing and four new taxon names are formally defined following the rules of the Phylocode.

The female atrial system of members of the Polycystididae has been studied at the light microscopical level and compared among the constituent taxa. Based on the criteria of position and conjunction, hypotheses of homology are put forward and compared with the assessments of homology found in earlier literature. Contradictory terminology is synonymized and adapted to fit new findings that recognize homology among structures in different taxa. Based on differences in position and structure, 2 types of female duct are recognized: female duct type I and female duct type II. The term “female bursa” is restricted to a sperm resorbing organ at the proximal end of the female duct type I and/or at the end of a common oviduct. Some species have bundles of glands at the place where the oviduct(s) enter the female duct type I, which are considered homologous among these species. Different types of seminal receptacles are recognized. The term “insemination duct” is restricted to the ducts which in some species, in addition to the oviducts, connect the ovaries with the female duct type I. A single muscular duct, the common oviduct, connects the joined oviducts with the female duct type I in species of Duplacrorhynchus and is considered homologous with the similar duct present in some other species. A uterus is always present, entering the common genital atrium through its frontal wall, or entering a female duct type II. This morphological review of the female atrial system reveals a mosaic distribution of characters if applied to previous phylogenies that are based primarily on the male reproductive system.

Acrorhynchides styliferus sp.n. from Norway and the British Isles and Polycystis contorta sp.n. and Typhlopolycystis coomansi sp.n. from Norway and the Mediterranean are described. Additional notes are given on the anatomy and distribution of Paracrorhynchus bergensis Karling, Polycystis groenlandica (Levinsen), P. crocea (Fabricius) and Typhlopolycystis mediterranea Brunet. The taxonomy of the species and the corresponding genera is discussed.

Paraustrorhynchus gen.n., P. pacificus sp.n. (California), Antiboreorhynchus novzelae sp.n. (New Zealand) and Gyratrix proaviformis sp.n. (Oregon) are described as new taxa. Additional notes are given on Gyratrix hermaphroditus Ehrbg. (new localities on the N. American Pacific coast), Scanorhynchus forcipatus Karling (new locality in California), Paraustrorhynchus elixus (Marcus) comb.n. and Alcha evelinae Marcus (new localities in California). The taxonomy of the genera and species is discussed. Viewpoints are presented on evolutionary trends and homologous atrial organs in the family Polycystididae.

The genital organs in adult specimens of Cirrifera aculeata have been studied by transmission electron microscopy. The male copulatory organ is of the conjuncta-duplex type provided with a spiny cirrus. The structure of the epithelia lining the different parts of the genital organs is described. The cirrus spines and the spines in the genital atrium are shown to be intracellular specializations. The structure and location of the cirrus spines in the Proseriata studied so far are discussed.