leeches continued

Leeches are presumed to have evolved from certain Oligochaeta, most of which feed on detritus. However, some species in the Lumbriculidae are predatory and have similar adaptations as found in leeches. Consequently, the systematics and taxonomy of leeches is in need of review. While leeches form a clade, the remaining oligochetes are not their sister taxon but a diverse paraphyletic group containing some lineages that are closely related to leeches, and others that are far more distant.

There is some dispute as to whether Hirudinea should be a class itself, or a subclass of the Clitellata. The resolution mainly depends on the eventual fate of the oligochaetes, which as noted above do not form a natural group as traditionally circumscribed. Another possibility would be to include the leeches in the taxon Oligochaeta, which would then be ranked as a class and contain most of the clitellates. The Branchiobdellida are leechlike clitellates which were formerly included in the Hirudinea but are apparently just really close relatives.

This giant Americobdella leech from southern Chile is an ancient arhynchobdellid. It is a predator, feeding on earthworms which it swallows whole.

Leech climbing a door by Lake Leake, Tasmania

The more primitive Acanthobdellidea are often included with the leeches, but some authors treat them as a separate clitellate group. True leeches of the infraclass Euhirudinea have both anterior and posterior suckers. They are divided into two groups: Arhynchobdellida and Rhynchobdellida

• Rhynchobdellida): "Jawless" leeches, armed with a muscular straw-like proboscis puncturing organ in a retractable sheath. The Rhynchobdellae consist of two families:
  • Glossiphoniidae: Flattened leeches with a poorly defined anterior sucker
  • Piscicolida: have cylindrical bodies and a usually well-marked, bell-shaped, anterior sucker. The Glossiphoniidae live in fresh-water habitats; the Pisciolidae are found in seawater habitats.

• Arhynchobdellida: Leeches which lack a proboscis and which may or may not have jaws armed with teeth. Arhynchobellids are divided into two orders:
  • Gnathobdela: In this order of "jawed" leeches, armed with teeth, is found the quintessential leech: the European medical (bloodsucking) leech, Hirudo medicinalis. It has a tripartite-jaw filled with hundreds of tiny sharp teeth. The incision mark left on the skin by the European medical leech is an inverted Y inside a circle. Its North American counterpart is Macrobdela decora, a much less efficient medical leech. Within this order, the family Hirudidae is characterized by aquatic leeches and the family Haemadipsidae by terrestrial leeches. In the latter are Haemadipsa sylvestris, the Indian leech and Haemadipsa zeylanica (yamabiru), the Japanese mountain or land leech.
  • Pharyngobdella: These so called worm-leeches consist of freshwater or amphibious leeches that have lost the ability to penetrate a host's tissue and suck blood. They are carnivorous and equipped with a relatively large, toothless, mouth to ingest worms or insect larvae, which are swallowed whole.
    The Pharyngobdella have six to eight pairs of eyes, as compared with five pairs in Gnathobdelliform leeches, and include three related families. The Erpobdellidae are some species from freshwater habitats.