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 clitellates are themselves a clade - have not been fully elucidated. For example, the "true leeches" (Euhirudinea) might actually be synonymous with the Hirudinea, as all other leech-like annelids might not be very closely related to the true leeches.
Proboscisless leeches are generally freshwater or amphibious animals. They have usually 6-8, sometimes 5 pairs of eyes. Some - among them the well-known European Medical Leech (Hirudo medicinalis) of the Hirudiniformes - are bloodsuckers, while most - including the Kinabalu Giant Red Leech (Mimobdella buettikoferi) of the Erpobdelliformes - are predators that hunt small invertebrates which are swallowed whole.
Systematics and taxonomy
Given the systematic uncertainties, the proboscisless leeches have been treated at varying ranks, and the alternate name Arhynchobdellae is also sometimes found.
Arhynchobdellida systematics has been revised in modern times. Formerly it was believed that they can be divided into a jawed and a jawless order. However, the lack of jaws does not denote a particularly close relationship. Jawed leeches are more numerous but exclusively found among the Hirudiniformes, while the jawless "Pharyngobdellida" (or "Pharyngobdellae"), also known as "worm-leeches", are more diverse and occur in the Hirudiniformes and the Erpobdelliformes alike.
In addition to the two orders accepted today, there is a more basal lineage. These, the Americobdellidae, are not assigned order rank but treated as a family to signify their phylogenetic position near the base of the Arhynchobdellida. They are massive annelids, measuring 30 cm long in some cases, and are anatomically primitive. These animals hunt earthworms, which they suck in one piece through their toothless mouth.
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Siddall, M.E. & Borda, E. (2004): Leech collections from Chile Including Two New Species of Helobdella. American Museum Novitates 3457: 1-18. PDF fulltext