 The two snails circle around each other for up to six hours , touching with their tentacles , and biting lips and the area of the genital pore , which shows some preliminary signs of the eversion of the penis . As the snails approach mating , hydraulic pressure builds up in the blood sinus surrounding the organ housing the dart . Each snail manoeuvres to get its genital pore in the best position , close to the other snail 's body . Then , when the body of one snail touches the other snail 's genital pore , it triggers the firing of the dart . 
