Most segmented worms, or annelids, eat decaying plant matter. The exceptions are leeches, freshwater animals that feed on blood and small invertebrates, and some carnivorous marine species.
Annelids share a bilaterally symmetric body plan of an elongated, cylindrical body divided into ring-shaped segments by internal barriers called septa. Each segment contains a complete set of organs. Annelids have well-developed nervous and circulatory systems. Most have the ability to regenerate lost body parts.
Earthworms are the most familiar annelids and are highly beneficial to plants and to other animals. They break down organic matter and return it to the soil in a form plants can use. Their tunnels help aerate the soil and allow air, water and nutrients to reach plant roots. In addition, they furnish food for many other animals.
Leeches are also familiar annelids. Like earthworms, they provide food for many other animals, and predatory species help keep worms, snails and aquatic insects in balance with their environments. Of the parasitic species that feed on blood, the medicinal leech, Hirudo medicinalis, helps people by removing excess blood from body parts after reattachment surgery and by stimulating blood flow. Medicinal leeches also supply the anticoagulant hirudin, and scientists are investigating the possibility that leeches may be useful in treating osteoarthritis as well.