Leeches - ancient blood-feeding creatures now approved as "medical devices"
Leeches are segmented worms belonging to the phylum Annelida. They're called segmented worms because their bodies are composed of ring-shaped segments (annuli), stacked one upon another (kind of like the Michelin Man, but with no arms or legs and quite a bit more slime).
There are around 650 species of leeches (out of 12,000 annelid species) and they belong to the class Hirudinea. Most leeches use a pair of sucker disks to attach themselves to surfaces that sometimes include prey animals. They also use these structures to "inch-worm" their way across the ground. Many leeches are excellent swimmers and there's even a Trinidadian leech that has reverted to the burrowing lifestyle of its ancestors.
Many leech species are blood feeding parasites (feeding on every group of vertebrates), while others are predators. This distinction (between parasite and predator) is sometimes a bit fuzzy, but one way to think of it is that predator/prey relationships are transient, with the prey ending up dead while parasite/host relationships are longer-term, with the host often surviving - though weakened). Some leeches are actually beneficial to the creatures they associate with. For example, there are leeches that inhabit on the exoskeletons of crayfish, where they survive by grazing on the micro-fauna (e.g., algae) growing on these mini-lobsters.
In Dark Banquet: Blood and the Curious Lives of Blood-Feeding Creatures, Bill Schutt spends an entire chapter dealing with the bizarre habits of leeches, especially their early medicinal use (e.g., Lord Byron, Joseph Stalin and James Joyce were all bled by leeches).
Additionally, there is extensive coverage of the leeches' successful return as an FDA approved "medical device". Readers will learn how modern surgeons use leeches after reattachment surgeries (e.g., ears and fingers). They do so because medicinal leeches establish a unique form of artificial venous circulation by removing the blood that typically accumulates in a surgically reattached structure (blood that would normally drain via the delicate and difficult-to-mend veins). The feeding leeches actually prevent blood from "pooling" at the reattachment site - a problem that can lead to blood clots and eventually, the death of the reattached tissue. The leeches function to buy time for the body's repair processes - which act to mend or replace the delicate veins, thus allowing blood to be drained from the reattached structure. At that point, leech therapy is halted. Depending on the complexity of the reattachment, hundreds of leeches may be used during a course of treatment.
Then there's actress Demi Moore, who recently told talk show host David Letterman that she uses "highly trained" medicinal leeches to detoxify her blood. 'kay.
Of the roughly 650 species of leeches, some are predatory, feeding on a variety of invertebrates (e.g., snails) while other species feed on the blood of vertebrates such as amphibians, reptiles, waterfowl, fishes, and mammals (including humans). In the 19th Century millions of leeches were used each year to draw blood (for every conceivable malady). After falling into disuse after the first decade of the 20th Century, leeches are now employed after reattachment surgeries (since they set up a sort of artificial venous circulation).