Comment by nycticorax

My PhD research was actually studying the leech nervous system. They're still an important 'model' organism in neurobiology. Probably not as important in the field at large as they were in, say, the 1970s, but still. They're also a good system for neurophysiology education, because they are cheap and easy to obtain, have large-ish neurons that are identifiable from animal to animal, and their nervous system has a relatively simple organization.