Dart sac
 Love dart which has pierced through the head of a snail – New Scientist
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Love darts of Cepaea nemoralis in action
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Love dart of Helix aspersa in action
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A love dart (also known as a gypsobelum) is a sharp, calcareous or chitinous dart which some hermaphroditic land snails and slugs create. Love darts are made in sexually mature animals only, and are used as part of the sequence of events during courtship, before actual mating takes place. Darts are quite large compared to the size of the animal: in the case of the semi-slug genus Parmarion, the length of a dart can be up to one fifth that of the semi-slug's foot.
The process of using love darts in snails is a form of sexual selection. Prior to copulation, each of the two snails (or slugs) attempts to "shoot" one (or more) darts into the other snail (or slug). There is no organ to receive the dart; this action is more analogous to a stabbing, or to being shot with an arrow or flechette. The dart does not fly through the air to reach its target however; instead it is fired as a contact shot.
The love dart is not a penial stylet (in other words this is not an accessory organ for sperm transfer). The exchange of sperm between both of the two land snails is a completely separate part of the mating progression. Nevertheless, recent research shows that use of the dart can strongly favor the reproductive outcome for the snail that is able to lodge a dart in its partner. This is because mucus on the dart introduces a hormone-like substance that allows far more of its sperm to survive.
Love darts, also known as shooting darts, or just as darts, are shaped in many distinctive ways which vary considerably between species. What all the shapes of love darts have in common is their harpoon-like or needle-like ability to pierce.
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Wikipedia