More than 15,000 species of annelids exist, including earthworms, leeches and polychaetes, which include fireworms, clam worms, lugworms and bristleworms. Earthworms are probably the most commonly known type of annelids; there are more than 2,700 species of earthworms, with the longest species capable of growing up to 22 feet long.

All species that belong to the phylum Annelida are invertebrate worms with segmented bodies, which allow them to be incredibly flexible and mobile. The majority of annelids tend to live in either soil or freshwater, although there are a few species that live on, rather than in, the ground.

The digestive system of annelids consists of a single tube that runs the length of the worm, with the mouth at one end and the anus at the other. Most annelids feed by passing soil through their bodies, absorbing nutrients from microscopic organisms in the soil. However, there are also parasitic annelids that feed off of a host organism, with blood-sucking leeches being the most common example.

Earthworms exist in huge numbers all around the world in virtually any soil that isn't frozen. It has been estimated that one acre of soil typically contains as many as 1 million of them.