Hagfish are marine craniates of the class Agnatha or Myxini, also known as Hyperotreti. Some researchers regard Myxini as not belonging to the subphylum Vertebrata. That is, they are the only animals that have a skull but not a vertebral column. Despite their name, there is some debate about whether they are strictly fish (as there is for lampreys), since they belong to a much more primitive lineage than any other group that is placed in the category of fish (Chondrichthyes and Osteichthyes). The earliest fossil record dates back approximately 330 million years to the Late Carboniferous period. Their unusual feeding habits and slime-producing capabilities have led members of the scientific and popular media to dub the hagfish as the most "disgusting" of all sea creatures.[ Although hagfish are sometimes called "slime eels," they are not eels at all.

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