Radix sp., showing copulation. The top snail is inserting its preputium (with penis at the preputium's tip) into the female gonopore of the bottom snail. The female gonopore is at the posterior of the "neck" stalk. The male gonopore is behind the right tentacle. During copulation, the preputium enverts carrying the penis at it enverted tip. Turgor pressure forces the preputium and penis out; penial retractor muscles pull it back after copulation. The vas deferens can be seen inside the preputium.
John B. Burch (photographer; copyright holder), Mollusk Division, Museum of Zoology, University of Michigan-Ann Arbor