When filled with blood vessels, the parapods increase the worm's surface area for gas exchange. Hemoglobin-filled gills allow some polychaetes to live in stagnant mud where food is plentiful but oxygen is scarce. A few species have taken up living as a commensal, living symbiotically on the bodies of other animals. Oligochaetes Annelids of the oligochaete line are almost all found in fresh water or terrestrial habitats. Students often can find the worms seen here in samples of pond water, which allows them to study these organisms in detail.
Earthworms are classic subjects for studying annelid anatomy.
Rather than dissecting a preserved earthworm, this program shows how to observe the organs of a small living earthworm using a microscope. The engulfment of soil by worms cultivates and aerates the soil, creating conditions suitable for plants to germinate and grow. Leeches Leeches, class Hirudinea, are offshoots of the oligochaete line. Some are scavengers or prey on small water animals. Many are ectoparasites of fish, and most infamously, a few kinds are blood-sucking parasites of mammals.