Leech Background
Phylum: Annelida (segmented worms)
Class: Hirudinea (leeches)
The leech is a member of the phylum Annelida or segmented worms. These worms are characterized by having bodies divided into similar parts, or segments. Most people are probably not very familiar with the most diverse group of this phylum, the bristle worms (class Polychaeta, over 4,000 species) which are all marine. On the other hand, most people are familiar with earthworms (class Oligochaeta, over 2,500 species). Leeches belong to class Hirudinea, a minority (over 300 species) in this phylum. They are characterized by having relatively few segments and having suckers at both ends of the body.
Many leeches are actually predacious, feeding on worms, snails and insect larvae, which are often swallowed whole. About three quarters of the known species are blood-sucking parasites, and these attack a variety of hosts including invertebrates, fish, amphibians, reptiles, birds, and mammals. These leeches are usually restricted to one class of vertebrates for hosts, so, for example, a leech which normally feeds on reptiles will rarely attack a mammal.
The leech attacks by attaching to a thin area of the host's skin and slicing it. The incision is anesthetized so the host is generally not aware of the attack, and the secretion of hirudin prevents coagulation of the blood. Leeches feed infrequently, but ingest a large quantity of blood per meal. The medicinal leech Hirudo medicinalis can ingest an amount of blood 2 to 5 times its own weight. The medicinal leech may require up to 200 days to digest a meal, and have been known to survive up to a year and a half without feeding.
The nervous system of the leech consists of the brain, the ventral nerve cord, and ganglia that are located in each segment along the nerve cord. The medicinal leech has 21 segmental ganglia, each containing 175 pairs of neurons. The relatively small number and the large size of the neurons have made leeches favorite subjects of neurobiologists.