 Fertilization is usually external . Typically , a short stage lasts a few hours or days before the eggs hatch into trochophore larvae . These later develop into veliger larvae which settle on the seabed and undergo metamorphosis into juveniles that are sometimes ( for example in the case of oysters ) known as " spat " . In some species , such as those in the genus Lasaea , females draw water containing sperm in through their inhalant siphons and fertilization takes place inside the female . These species then brood the young inside their mantle cavity , eventually releasing them into the water column as veliger larvae or as crawl @-@ away juveniles . 
