Ok, so the way snail sex works is madness-inducing. When gastropods moved on land, they became hermaphroditic so it was easier to find compatible mates, since they move so slow and you can't broadcast gametes outside of water, which is sensible enough.
But here's how it works. The ovotestis, embedded in the hepatopancreas, is simultaneously an egg-producing ovary and a sperm-producing testicle. Newly produced eggs travel to the spermatheca to await fertilization. Sperm also goes to the spermatheca, travels further along the vas deferens which is embedded in the oviduct but separated from it, into the epiphallus. Here it is mixed with the protein coating secreted by the flagellum, forming the spermatophore. The fully coated spermatophore goes into the penis, which turns inside-out like a glove finger during copula to go into the partner.
At the same time, the partner's spermatophore is received into the vagina, travels along a narrow duct toward the bursa copulatrix where the spermatophore coating is digested, and then the released sperm travels back along the same duct to go into the oviduct and then the spermatheca, where it can fertilize the eggs. At that point the fertilized eggs are coated in albumen by the relevant gland and go down the oviduct and the vagina to be laid out.
Oh, and there's also the "love dart", a lubricated calcareous spike that stabs the partner during mating, not to transfer gametes but hormones that make it more receptive to the spermatophore.
I prepared this diagram for a zoology lecture, and it took me and my tutor a full hour to reconstruct it and map it onto the didactic model.