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  Sixgill hagfish
(Eptatretus hexatrema)

The hagfish family, Myxinidae, is the only family in the order Myxiniformes (also known as Hyperotreti), which itself is the only order in the class Myxini. Thus, hagfish is variously used for any of the three taxonomic levels

Hagfish are jawless and generally classified with the lampreys into the superclass Agnatha (jawless vertebrates) within the subphylum Vertebrata. However, hagfish actually lack vertebrae. For this reason, they sometimes are separated from the vertebrates and not even considered to be fish. Janvier (1981) and a number of others put hagfish in a separate subphylum Myxini, which along with the subphylum Vertebrata comprises the taxon Craniata, recognizing the common possession of a cranium (Janvier 1981). Others, however, place Vertebrata and Craniata as synonyms at the same level of classification, and thereby retain hagfish (Myxini) as members of the superclass Agnatha within the vertebrates (Nelson 1994). The other living member of Agnatha, the lamprey, has primitive vertebrae made of cartilage.

Yet other classifications place Myxini as a class that in one instance lies within the subphylum Vertebrata (ITIS 2003) and, in another instance, lies within—and is the only class in—the clade Craniata, which is considered to be separate from the subphylum Vertebrata (Campbell and Reece 2005).

As members of Agnatha (Greek, "no jaws"), hagfish are characterized by the absence of jaws derived from gill arches (bone or cartilage supporting the gills), although they do have a biting apparatus that is not considered to have been derived from gill arches (Nelson 1994). Other common characteristics of Agnatha include the absence of paired fins, absence of pelvic fins, the presence of a notochord both in larvae and adults, and seven or more paired gill pouches. In addition, the gills open to the surface through pores rather than slits, and the gill arch skeleton is fused with neurocranium (the portion of the skull the protects the brain) (Nelson 1994).

Despite their name, there is some debate about whether hagfish are strictly fish, since they belong to a much more primitive lineage than any other group that is commonly defined as fish (Chondrichthyes and Osteichthyes), and because of the lack of a vertebrae commonly associated with the definition of fish. However, many authors do place hagfish as a primitive fish.

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