Euhirudinea, the true leeches, are an infraclass of the Hirudinea. These clitellate

Clitellata

Clitellata is a class of Annelid worms, characterized by having a clitellum - the 'collar' that forms a reproductive cocoon during part of their life cycle. The clitellates comprise around 8,000 species...

 annelid

Annelid

The annelids , formally called Annelida , are a large phylum of segmented worms, with over 17,000 modern species including ragworms, earthworms and leeches...

s are of somewhat unclear relationships; namely the relationships of Hirudinea with oligochaetes are in need of revision. It may be that the presumed sister taxon of the Euhirudinea, the Acanthobdellidea, turns out to be more distantly related, as was already the case with the Branchiobdellida. Thus, eventually Euhirudinea might become a junior synonym of Hirudinea.

True leeches are subdivided into two groups, the Arhynchobdellida

Arhynchobdellida

The proboscisless leeches, Arhynchobdellida, are classified as an order of the Hirudinea. But leech taxonomy and systematics will eventually be revised in due time, not because there are many uncertainties about their phylogeny, but because the major clades of clitellate annelids - and whether the...

 or Arhynchobdellae (proboscisless leeches), and the Rhynchobdellida

Rhynchobdellida

Rhynchobdellida, the jawless leeches, are classified as an order of the Hirudinea. But leech taxonomy and systematics will eventually be revised in due time, not because there are many uncertainties about their phylogeny, but because the major clades of clitellate annelids – and whether the...

 or Rhynchobdellae (jawless leeches). Note that the lack of jaws is a plesiomorphy, while the presence of a proboscis is an apomorphy - not all Arhynchobdellida have jaws, but all Rhynchobdellida have a proboscis. The most well-known leech species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

, Hirudo medicinalis (European Medical Leech), belongs to the Arhynchobdellida.