These animals are ectoparasites. This means that they use their chitin-containing jaws to attach to victims. To keep the blood flowing freely and to prevent clotting leeches inject anticoagulants into the wound. An antibiotic is also added to the blood. Leeches eat only once every month or so.
Undigested blood remains in the digestive tract of the leech for long periods of time.
The antibiotic prevents the blood from spoiling. People cultivate earthworms for several purposes. One example is the use of earthworms as fish bait. The are also used in gardening. In their release of urea and nutrients such as calcium into the soil, earthworms also enrich the soil.