Habitat

For the most part, annelids are found everywhere. And really, everywhere on the face of the planet that has liquid water or mud or dirt will probably have some kind of worm living there.

Being very low on the food chain, the presence of annelids in a given area can usually indicate that there is, or will be, another speicies which feeds on them also present in the same area. On land, these worms also tend to dig burrows and tunnels underground, which ends up helping any surrounding plant life by allowing rain a more direct path to the roots.

Another explaination for their widespread distribution comes from the early point of evolution these little critters occupy; because of their diversity that resulted from early evolving, they spread very far over the various faces of the land and sea.