The tiny larva knows it's running out of energy, and its antennae are frantically twitching. It can't feed until it has found a home. A current starts to pull on it. The larva instinctively knows something is wrong, and desperately tries to swim against it. It's sucked into a black abyss, and then the current stops. Larva antennae feel something, and grab it. Cement glands immediately extrude a glue, and the larva is now securely attached by its head. Now it feeds.
The one eye knows it's dark, but limbs reach out and snatch food. Secreting calcium carbonate, the larva bonds even more firmly to the unwilling host. The current starts again, but it can no longer budge the larva. It has built solid walls around it. The flowing water provides food, and it grows. Now secure, it creates offspring and sets them free to infest the new unwilling host.