The ultrastructure of the proboscis in <i>Psammorhynchus tubulipenis</i> Karling, 1964 and <i>Cytocystis clitellatus</i> Karling, 1953 (Platyhelminthes, Rhabdocoela)

The ultrastructural organization of the proboscis in two species of free-living Platyhelminthes, Psammorhynchus tubulipenis and Cytocystis clitellatus is very alike but differs from previously described species. Both sheath and cone epithelium are composed of two circumferential belts. Only the basal cone epithelium is syncytial, while no nuclei were found in the distal belt of the sheath epithelium. The sheath epithelium is characterized by numerous infoldings of the basal plasma membrane. The nuclei present in the bulb belong to the proximal belt of the sheath epithelium and the apical cone epithelium. Nuclei of the basal cone epithelium are located insunk behind the proboscis bulb. The insunk cell parts pierce the septum of the bulb laterally near the proximal end. Different types of gland necks and sensory cells pierce the epithelia. Associated with the distal belt of the sheath epithelium, two sensory organs are found, containing multiciliary receptors with modified axonemata. Differences in organization of the proboscis musculature are described and compared with the organization in other species. The systematic position of both species is discussed in the light of the new findings