Annelids are segmented worms that are characterized by their bilateral symmetry (as opposed to the radial symmetry found in lower animals) and are segmented. They are interesting to study since phylogenticaly they are somewhere between earliest multi cellular animals and the very complicated insects. Annelids have closed circulatory systems. The exchange of oxygen and carbon dioxide occurs on its moist skin.
Since there are only 26 species of annelids and most of them are aquatic, Annelid collection and identification will be limited to earthworms. We will use them primarily as a soil health indicator. When you find them in sufficient numbers in a particular area you can be assured the soil has a recent history of sufficient moisture content. They are beneficial to an ecosystem because they loosen the soil thereby increasing air and water penetration. Darwin estimated that an acre of British farmland contained 50,000 earthworms that produced 18 tons of castings per year. They also provide a robust meal for some of the bird populations that will visit the neighborhood. It is important for the students to recognize that the various insect larvae they will find latter on are not worms