Leeches are worms of the subspecies Hirudinea that live in oceans, rivers, or on land. They consist of several parts or segments; a front area designated as the head or anterior brain, the middle part consisting of segments each containing a nerve ganglion as well as other organs, and the hind part which has the posterior brain and the tail sucker. Most leeches use their suckers to feed on the blood of their hosts, hence their traditional use in medicine. The print shown here is from a volume containing thousands of illustrations by Gustaf Retzius in a publication titled Biologische Untersuchungen. In this plate Retzius shows the nervous system of the medicinal leech in ethereal color. He was the publisher and usually sole contributor to his own publications.