In the paper the author describes the morphology and anatomy of the Rissostomia species inhabiting Puck Bay. A detailed description of the shell and its variability (including the colour spots and ribs occurring very rarely) is given. Then the author describes the head pigmentation, variable and quite uncharacteristic. The concise descriptions of the soft parts morphology, ctenidium, radula and male reproductive organs are given. The variability of the giant penis is described, as well as the structure of the female reproductive organs. Then the author compares the Polish Rissostomia with the drawings and descriptions of R. membranacea given in the literature, as well as with the specimens of the species from the localities outside the Baltic Sea. He establishes that the Polish Rissostomia belong to Rissoa membranacea which is an extremely polymorphic species.