【英语时差8,16】蜗牛2（在线收听）

Now here’s the strange part. About once a week, as garden snails share caresses, pressure builds up in the area surrounding a sac near the genital region housing a calcium dart. Just for the moment of sexual penetration, the impregnating snail stabs its partner near the genitals with what scientists have dubbed the snail love dart. But what might sound like a nasty sadomasochistic ritual does have a practical purpose. According to one study, the darts are tipped with a chemical preventing the snail on the receiving end from digesting most of its mate’s sperm. As in many species, the garden snail’s female reproductive tract is hostile to sperm, allowing only the toughest and most resilient to fertilize the egg. To increase its chances of passing on its genes, the impregnating snail fires a dart to give its sperm a fighting chance.