Annelids, or segmented worms, are important because some of them aerate and enrich soil, which keeps it fertile. Medicinal leeches are annelids that are used in healing.
An earthworm eats the soil it burrows through and extracts nutrients from both living and dead material. But earthworms' waste also adds nutrients to the soil, and their burrowing breaks up the soil enough to allow oxygen and water to enter it. The earthworm also makes excellent fishing bait and is food for animals such as birds and rodents.
The medicinal leech can be found in freshwater, although it can live on land. This animal was used routinely by doctors to bleed their patients. Though bleeding fell out of favor, the medicinal leech is still used as of 2014 to help heal severed appendages and in cosmetic surgeries and skin grafts. To make sure that it can have a good blood meal and not be immediately detected by its host, the leech has evolved powerful anesthetics, antibiotics and anticoagulants that scientists are keen to study.
Annelids get their name from the fact that their bodies are made up of ringed segments. They have a mouth at one end that is attached to a long gut that terminates at the anus at the other end.