Leech (class Hirudinea) is a segmented annelid worm with 34 segments, many external rings and no setae (bristles). Suckers at each end are used in looping movements. Many aquatic leeches can also swim.


The gut has a large storage area to hold massive blood meals needed for a balanced diet; the intestine is simple, as blood is easily digested and absorbed. Some leeches eat detritus; others, soft-bodied animals with red blood (eg, snails, worms). Most are external parasites on vertebrates. Some may invade the bladder or body parts near the exterior, eg, nostrils.

Distribution and Habitat

About 300 species are known worldwide, most from freshwater habitats; some are marine or terrestrial, especially in tropical areas. There are some 45 Canadian species, if those known from bordering US are included.

Biological Importance

The traditional medicinal use of leeches to draw blood continues, eg, in draining blood from reattached, severed fingers before natural circulation is fully restored.