Leeches: Hirudinea

Many leeches swim through the water with snakelike motions. They release their grip with the tail sucker and push off from a rock or plant, before wriggling their body back and forth. Species living on land move along the ground like an inchworm, stretching and shortening their bodies by using the suckers on both ends.

Leeches must mate to reproduce. Mating occurs when a leech attaches a sperm packet in the body of its mate. Some species have corresponding male and female organs that allow the placement of sperm directly into their mate's female reproductive organs. Fertilization (FUR-teh-lih-ZAY-shun) occurs inside the female's body. As the cocoon passes over the female reproductive organs, the developing eggs, or embryos (EHM-bree-ohz), are deposited inside.

The cocoons are either left in the soil, or attached to the bodies of other animals. Young leeches resemble the adult when they hatch. Fish leeches attach their egg cases to the bodies of crustaceans (krus-TAY-shuns), or water-dwelling animals with soft bodies covered by hard shells, such as shrimp or lobster. When fishes eat infested crustaceans, the young leeches attach themselves inside the fishes' mouth cavity. Some leeches carry their eggs in a clear sack under their body. When the adult finds a host, like a turtle or frog, the young hatch and also attach themselves to the host.