1: substantial medial, dorsal ridge.
2: fixed, erect rim of the mantle.
3: extended penis with fine brown marks basally.
This pair are in the head to tail mating position which brings adjacent their right sides bearing the hermaphrodite, genital openings. In this case, not closely adjacent, but the penis of one has extended to reach the opening of the other (4).
Sublittoral, Oosterschelde, Nederland. © S. Verheyen.