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|Cystobranchus respirans on a common roach|
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 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.
Compared to their sister taxon, the Arhynchobdellida (proboscisless leeches), rhynchobdellid systematics has always been rather straightforward as regards the main clades contained therein. The common name is actually misleading however, as jawlessness is a primitive trait among leeches in general – most Arhynchobdellida are in fact jawless too. It is rather the proboscis that is the characteristic apomorphy of the Rhynchobdellida.
Two or three families are recognized: The Glossiphoniidae are freshwater leeches, flattened, and with a poorly defined anterior sucker. The Piscicolidae occur in both freshwater and seawater, have cylindrical bodies, and a usually well-marked, bell-shaped, anterior sucker. The Ozobranchidae are not universally accepted as distinct; some authors prefer to merge them with the Piscicolidae.
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