Hagfish

Hagfish
Pacific hagfish resting on bottom 280 m depth off Oregon coast
Scientific classification
Kingdom:	Animalia
(unranked):	Craniata
Superclass:	Cyclostomata
Class:	Myxini
Order:	Myxiniformes
Family:	Myxinidae

Hagfish are craniates in the superclass Cyclostomata, class Myxini. Hagfish do not have a skeleton, except they do have a skull, which is made of cartilage.

Because of this, many researchers think Myxini should not be in the subphylum Vertebrata.^[1] However, because of its fins and gills, they are called fish. They are marine, meaning they live in the sea.

The original 19th century classification groups hagfish and lampreys together as cyclostomes (or historically, Agnatha), as the oldest surviving class of vertebrates alongside gnathostomes . An alternative scheme proposed that jawed vertebrates are more closely related to lampreys than to hagfish, so vertebrates include lampreys but exclude hagfish.

Recent DNA evidence supports the original scheme.^[2][3]

1. ↑ Neil A. Campbell and J.B. Reece 2005. Biology. 7th ed, Benjamin Cummings, San Francisco CA.
2. ↑ Janvier P. (2010). "MicroRNAs revive old views about jawless vertebrate divergence and evolution". Proceedings of the National Academy of Sciences. 107 (45): 19137–19138. Bibcode:2010PNAS..10719137J. doi:10.1073/pnas.1014583107. PMC 2984170. PMID 21041649. Although I was among the early supporters of vertebrate paraphyly, I am impressed by the evidence provided by Heimberg et al. and prepared to admit that cyclostomes are, in fact, monophyletic. The consequence is that they may tell us little, if anything, about the dawn of vertebrate evolution, except that the intuitions of 19th century zoologists were correct in assuming that these odd vertebrates (notably, hagfishes) are strongly degenerate and have lost many characters over time
3. ↑ Heimberg A.M. et al 2010. microRNAs reveal the interrelationships of hagfish, lampreys, and gnathostomes and the nature of the ancestral vertebrate. Proc Natl Acad Sci USA 107:19379–19383. [1]