Why Are Earthworms Unable to Self-Fertilize?

A common misconception is that, because earthworms have both male and female sex organs, they can self-fertilize. However, earthworms? sex organs grow at opposite ends of their bodies, making it impossible for one worm to fertilize its own eggs.

Like sea anemones and tapeworms, earthworms are simultaneous hermaphrodites. These species are usually sedentary, living entirely in one place. If they are mobile, they don?t range across large territories. When two simultaneous hermaphrodites meet, they mate and fertilize each other?s eggs.

When two earthworms meet coming from opposite directions, they align their sex organs so that one worm?s male organs connect to the other worm?s female organs. As fertilization occurs, both worms form and deposit eggs in a lemon-shaped slime tube that they then bury in the soil. In two to four weeks, little worms emerge from the buried tube. Except for their sex organs, which develop in 60 to 90 days, these small worms are fully formed.

Earthworms? copulation and reproduction are separate processes. Mating worms couple to collect each other?s sperm and then crawl off. Long after copulation, worms form their slime rings. To lay their eggs, they crawl backward out of their rings, depositing their own eggs and the other worm?s sperm. The slime rings seal completely once the worm is free, becoming a sort of cocoon.