Leech, or Bloodsucker, a bloodsucking annelid worm. Most leeches live in the sea. Some live in freshwater and others on land. A leech has an elongated body that is soft but tough and is usually blackish or brownish. The body is made up of numerous segments. The adult bears both male and female sex organs and reproduces by cross-fertilized eggs. Leeches range in length from less than 1 inch (2.5 cm) to 12 inches (30 cm) or more.

Some leeches feed mainly on blood, but others eat worms, insect larvae, and other small forms of animal life. One group of leeches attach themselves to fish; another to turtles and worms; a third, to snails and other leeches; a fourth, to warm-blooded land animals. Few leeches actually require blood as food, though most will accept this food if the chance arises.

Two important leeches are the horseleech and medicinal leech. The horseleech, which was once believed to attack the breathing passages of horses, is black and grows six to eight inches (15 to 20 cm) in length. The medicinal leech is cross barred with brown and black and grows to eight inches (20 cm) in length. The medicinal leech is native to Europe and has become naturalized in some streams of eastern North America.

In the 18th and 19th centuries, when bloodletting was a common treatment for disease, physicians used leeches to treat headache, arthritis, tuberculosis, lumbago, gout, and many other illnesses. Leeches are still used sometimes to clear up the discoloration of black eyes and to keep capillaries open after severed extremities have been surgically reattached.

The medicinal leech has a sucker at each end of the underside of its body. The sucker at one end contains the animal's mouth. The other sucker acts as a suction cup when the leech attaches itself to its victim. Once attached, the leech slashes its victim's skin with its sharp teeth and sucks the blood. The leech injects into the wound a substance that keeps blood from clotting. A leech can ingest up to five times its weight in blood and then live up to 18 months without food.

Leeches belong to the class Hirudinea of the phylum Annelida. The horseleech is Haemopis gulo; medicinal, Hirudo medicinalis.