Euhirudinea, the true leeches, are an

infraclass In biological classification, class ( la, classis) is a taxonomic rank, as well as a taxonomic unit, a taxon, in that rank. It is a group of related taxonomic orders. Other well-known ranks in descending order of size are life, domain, kingdom ...

of the

Hirudinea Leeches are segmented parasitic or predatory worms that comprise the subclass Hirudinea within the phylum Annelida. They are closely related to the oligochaetes, which include the earthworm, and like them have soft, muscular segmented bodie ...

Taxonomy

These

clitellate The Clitellata are a class of annelid worms, characterized by having a clitellum - the 'collar' that forms a reproductive cocoon during part of their life cycles. The clitellates comprise around 8,000 species. Unlike the class of Polychaeta, the ...

annelid The annelids (Annelida , from Latin ', "little ring"), also known as the segmented worms, are a large phylum, with over 22,000 extant species including ragworms, earthworms, and leeches. The species exist in and have adapted to various ecol ...

s are of somewhat unclear relationships; namely the relationships of Hirudinea with

oligochaete Oligochaeta () is a subclass of animals in the phylum Annelida, which is made up of many types of aquatic and terrestrial worms, including all of the various earthworms. Specifically, oligochaetes comprise the terrestrial megadrile earthworms ...

s are in need of revision. It may be that the presumed

sister taxon In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and t ...

of the Euhirudinea, the

Acanthobdellidea Acanthobdellidea is an infraclass of primitive leeches; some authors place them in a separate subclass from the Hirudinea. However, the World Register of Marine Species places them within the Hirudinea, as a sister group to Euhirudinea, the true l ...

, turns out to be more distantly related, as was already the case with the

Branchiobdellida Branchiobdellida is an order of freshwater leech-like clitellates that are obligate ectosymbionts or ectoparasites, mostly of astacoidean crayfish. They are found in the Northern Hemisphere and have a holarctic distribution in East Asia, the Eu ...

. Thus, eventually Euhirudinea might become a

junior synonym The Botanical and Zoological Codes of nomenclature treat the concept of synonymy differently. * In botanical nomenclature, a synonym is a scientific name that applies to a taxon that (now) goes by a different scientific name. For example, Linna ...

of Hirudinea. True leeches are subdivided into two groups, the

Arhynchobdellida Arhynchobdellida, the proboscisless leeches, are a monophyletic order of leeches. They are defined by the lack of the protrusible proboscis that defines their sister taxon, the Rhynchobdellida.Uttam, Suneha, and Seema Langer.Distribution and Ide ...

or Arhynchobdellae (proboscisless leeches), and the

Rhynchobdellida Rhynchobdellida (from the Greek ''rhynchos'', mouth, and ''bdellein'', sucking), the jawless leeches or freshwater leeches, are an order of aquatic leeches. Despite the common name "freshwater leeches", species are found in both sea and fresh wa ...

or Rhynchobdellae (jawless leeches). Note that the lack of jaws is a

plesiomorph In phylogenetics, a plesiomorphy ("near form") and symplesiomorphy are synonyms for an ancestral character shared by all members of a clade, which does not distinguish the clade from other clades. Plesiomorphy, symplesiomorphy, apomorphy, and ...

y, while the presence of a proboscis is an

apomorph In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to have ...

y – not all Arhynchobdellida have jaws, but all Rhynchobdellida have a proboscis. The best-known leech

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

, ''

Hirudo medicinalis ''Hirudo medicinalis'', the European medicinal leech, is one of several species of leeches used as "medicinal leeches". Other species of ''Hirudo'' sometimes also used as medicinal leeches include '' H. orientalis'', ''H. troctina'', and '' H. ...

'' (European Medical Leech), belongs to the Arhynchobdellida.

Ecology

Euhirudinea leeches tend to hunt for nutrients in the morning or afternoon. This is significant because leeches that hunt in the morning are found in larger sizes than those who hunt in the afternoon. Therefore, feeding in the morning seems to be more nutritious for the leeches than in the afternoon. These leeches are found either on trail, or off-trail, this is important for there had been found a greater amount of leeches that are found off-trail, than those that are on-trail.

References

Leeches {{annelid-stub