Blood-letting by leeches has been a common medical practice in many parts of the world since ancient times. A leech has a large muscular sucker on the tail end and a smaller one on the head, around the sucking mouth which, in some kinds, is armed with jaws. They are very sensitive to shadows passing over them, to the slightest vibrations in the water, and to small changes in the flavor of the water about them. All leeches are contortionists. Some can stretch out as long and slender as a toothpick one moment, and be contracted into a tight ball the next.
Many kinds swim with a serpentine movement like miniature eels.
In leeches, as in earthworms, pairs of animals mate even though each one has a complete set of male and female organs. The eggs of one are fertilized by the sperm cells of the other. They lay their eggs in spring and summer, usually in a horny capsule or cocoon which is glued to stones, plants or trash. However, one family of leeches fastens these egg capsules to their undersides, where the young hatch out and cling to the parent with their heads hanging free. Their feeding habits are curiously varied.