The sex of active adult individuals can often be determined, if the shell is translucent and not obscured by epizoic growth, as the large penis is visible as a broad whitish band, often, with a blackish line formed by the genital duct (1). Females lack the band and, instead, irregular dark and pale marks on the body are visible (2). N.B. the penis cannot be seen through the body whorl if the animal is retracted into the shell; see image 03 Rissoa membranacea.