Abstract
The taxonomic status of many species of the family Cyclocoelidae is still unclear. Two species of cyclocoelids, Harrahium obscurum and Morishitium dollfusi, were collected from the air sacs of birds (Turdus merula and Tringa ochropus) inhabiting the Middle Volga region (European Russia). Here, we provide the first detailed morphological description of these cyclocoelids and combine it with the first molecular phylogenetic analysis of Cyclocoelidae from birds in Russia based on partial sequences of their 28s rDNA and coI mtDNA genes. Specimens of both flatworm species from different host individuals differ slightly in body shape and size, which probably reflects host-induced intraspecific variability. For the first time, we have shown that a stable morphological character such as the length of the vitelline fields in the studied digeneans is variable at the species level and cannot be used in their morphological diagnosis.