This paper contributes to the systematics, the ecology and the biology of sexual reproduction of psammobiontic macrostomid Turbellaria from a sandy beach on the eastern shore of the island Sylt. The anatomy of a new species, Paromalostomum proceracauda, is described. The cuticular structures of the genital apparatus of Paromalostomum fusculum Ax, P. dubium (de Beauchamp) and Haplopharynx rostratusMeixner are analyzed. P. fusculum is one of the most frequent turbellarians living in the sandy flats (Sandwatt) in front of the Litoralstation of the Biologische Anstalt Helgoland in List. This species shows obvious seasonal differences in it's vertical distribution. In summer, P. fusculum lives in the upper 4 cm of the sand while in winter, the animals move to the deeper strata of the sediment, were they are more protected from low temperatures. P. fusculum has an annual life-cycle. The period of reproduction lasts from May to July. The new generation hibernates in the state of male maturity and attains female maturity in spring. After the period of egg-shedding, large animals individuals are seldom found; it is supposed that they die after spawning. The mechanism of egg-shedding and the structure of the egg are described. The egg-shells of P. fusculum are distinguished from those of P. dubium by the presence of opercula. © 1969 Biologischen Anstalt Helgoland.
