Leeches, which are rather slimy worms and vary in length from an inch to several inches, have two suckers, a big one a the rear and a smaller one at the mouth end. They have powerful muscles which enable them to expand and contract their bodies.
This makes them excellent swimmers. They can also use their suckers to crawl on the land. In tropical Asia, the islands of the Pacific and the Indian Ocean, there is a particularly vicious and dreaded species of land leech which enters the breathing passages of animals, gorges on the animal's blood and swells so that it cannot escape.
Aquatic or water leeches cling to fishes, turtles and shell fish. Some leeches feed on earthworms and frogs' eggs. Others live on the larvae of insects and even on the microscopic life on the floor of the pond.
Leeches have been used in medicine from early times until quite recently to draw blood from a patient. They also serve as fish bait. In parts of the United States they are regarded as useful in controlling the snail population in lakes and ponds.