Description and Size
Leeches are segmented worms belonging to Annelida. They are usually flattened dorso-ventrally and have distinct suction disks (suckers) on either end of their bodies. They have a distinct head end with a mouth consisting of sharp cutting beaks located within the head sucker. Their rear sucker is larger and use to anchor the leech in place. Leeches have simple eyes which detect light and dark, rough shapes and motion. Color and pattern vary greatly from species to species, but most tend to be darker colored with blotches, spots or stripes.
Habitat
The majority of leeches live in fresh water habitats, but some inhabit salt water environments and some can live in moist terrestrial environments. They are hermaphrodites, possessing both sex organs in each individual, but they must cross-fertilize with another of their species to produce viable offspring. Two individuals will align themselves and inject each other with sperm. The resulting fertilized eggs are kept in a mucous cocoon which may be kept attached to the underside of the body or deposited in the environment.
Feeding
Food for most species of leeches is the blood of other animals; they are “hematophagous”. Blood-sucking leeches are ambush predators that wait for larger animals to swim by, or move close in the case of terrestrial species, then they grab on to their prey. Razor sharp blades or beaks in the mouth cut a slot through which blood flows aided by anticoagulants and anesthetics to dull sensations at the wound. Large volumes of blood, up to several times the weight of the leech, can be stored in a crop (a widening of the alimentary canal) before digestion begins. When the leech has filled its crop with blood it releases its suckers and falls off. A large blood meal can last a leech for many months. Some leech species eat other tissues on their prey, some are carrion feeders and some swallow small invertebrates whole. And lots of other critters will make a meal out of leeches: fish, turtles, shore birds, herons, etc. Leeches don’t have it all their own way, they’re just part of the food chain.
Medical Uses
Hirudo medicinalis, which are found all over the world, living mostly in fresh water, have long had a place in the doctor’s medical kit. Five thousand years ago, Egyptian medics believed that letting a leech sip a sick patient’s blood could help cure everything from fevers to flatulence. And in medieval Europe, leeches were so closely associated with doctors that physicians were called “leeches” — and they used millions of the parasites annually to treat patients.
In the 20th century, however, most doctors turned away from the worms, which in nature feed on everything from frogs to alligators. A few physicians, however, saw that leeches might play a special role in certain kinds of surgery, by helping promote blood flow to damaged tissue. That’s because when leeches bite a victim, their unique saliva causes blood flow to increase and prevents clotting. As a result, once bitten, victims can bleed for hours, allowing oxygenated blood to enter the wound area until veins re-grow and regain circulation.
The leech is invaluable in microsurgery when faced with the difficulties of reattaching minute veins. Ears have such tiny veins that, in the past, no one was able to successfully reattach them. Then, in 1985, a Harvard physician was having great difficulty in reattaching the ear of a five-year-old child; the tiny veins kept clotting. He decided to use leeches and the ear was saved. This success established leeches in the modern medical world. Since then, leeches have saved lives and limbs, reducing severe and dangerous venous engorgement post-surgery in fingers, toes, ear, and scalp reattachments; limb transplants; skin flap surgery; and breast reconstruction.