Editing Hagfish (section)

==Physical characteristics==
[[File:NIE 1905 Hagfish - Myxine glutinosa.jpg|thumb|left|Two views of the hagfish (''Myxini glutinosa'') with analytical overlays and dissection, published 1905]]

===Body features===
Hagfish are typically about {{cvt|50|cm|1}} in length. The largest-known species is ''[[Eptatretus goliath]]'', with a specimen recorded at {{cvt|127|cm|ftin}}, while ''[[Myxine kuoi]]'' and ''[[Myxine pequenoi]]'' seem to reach no more than {{cvt|18|cm}}. Some have been seen as small as {{cvt|4|cm}}.{{Citation needed|date=July 2022}}

Hagfish have elongated, eel-like bodies, and paddle-like tails. The skin is naked and covers the body like a loosely fitting sock. They are generally a dull pink color and look quite worm-like. They have [[cartilage|cartilaginous]] skulls (although the part surrounding the brain is composed primarily of a fibrous sheath) and tooth-like structures composed of [[keratin]]. Colors depend on the species, ranging from pink to blue-grey, and black or white spots may be present. Eyes are simple eyespots, not lensed eyes that can resolve images. Hagfish have no true fins and have six or eight [[barbel (anatomy)|barbel]]s around the mouth and a single nostril. Instead of vertically articulating jaws like [[Gnathostomata]] ([[vertebrate]]s with jaws), they have a pair of horizontally moving structures with tooth-like projections for pulling off food. The mouth of the hagfish has two pairs of horny, comb-shaped teeth on a cartilaginous plate that protracts and retracts. These teeth are used to grasp food and draw it toward the pharynx.<ref name=r1>[http://tolweb.org/Hyperotreti Hyperotreti] {{Webarchive|url=https://web.archive.org/web/20130206033343/http://tolweb.org/Hyperotreti |date=2013-02-06 }}. Tree of Life</ref>
[[Image:Eptatretus stoutii.jpg|thumb|[[Pacific hagfish]] at 150 m depth, [[California]], [[Cordell Bank National Marine Sanctuary]]]]

Its skin is attached to the body only along the center ridge of the back and at the slime glands, and is filled with close to a third of the body's blood volume, giving the impression of a blood-filled sack. It is assumed this is an adaptation to survive predator attacks.<ref>{{Cite web |last=Chodosh |first=Sara |date=14 December 2017 |title=The world's fastest shark is no match for a sack of flaccid hagfish skin |url=https://www.popsci.com/shark-hagfish-flaccid-skin-sack/ |access-date=29 August 2024 |website=[[Popular Science]] |language=en-US |archive-date=13 April 2024 |archive-url=https://web.archive.org/web/20240413072251/https://www.popsci.com/shark-hagfish-flaccid-skin-sack/ |url-status=live }}</ref> The Atlantic hagfish, representative of the subfamily Myxininae, and the Pacific hagfish, representative of the subfamily Eptatretinae, differ in that the latter has muscle fibers embedded in the skin. The resting position of the Pacific hagfish also tends to be coiled, while that of the Atlantic hagfish is stretched.<ref>{{Cite web |last=Pennisi |first=Elizabeth |date=6 January 2017 |title=How the slimy hagfish ties itself up in knots—and survives shark attacks |url=https://www.science.org/content/article/how-slimy-hagfish-ties-itself-knots-and-survives-shark-attacks |access-date=30 August 2024 |website=[[Science (journal)|Science]] |archive-date=15 June 2022 |archive-url=https://web.archive.org/web/20220615080958/https://www.science.org/content/article/how-slimy-hagfish-ties-itself-knots-and-survives-shark-attacks |url-status=live }}</ref><ref>{{Cite web |url=http://www.sicb.org/meetings/2017/schedule/abstractdetails.php?id=391 |title=Comparative Biomechanics of Hagfish Skins SICB - 2017 meeting - Abstract Details |access-date=2018-05-17 |archive-date=2018-05-21 |archive-url=https://web.archive.org/web/20180521021841/http://www.sicb.org/meetings/2017/schedule/abstractdetails.php?id=391 |url-status=dead }}</ref>

===Slime===
[[File:Hagfish Slime Predator Deterrence.jpg|thumb|An [[Myxine glutinosa|Atlantic hagfish]] (''Myxine glutinosa'') using its slime to get away from a [[kitefin shark]] (''Dalatias licha'') and an [[Atlantic wreckfish]] (''Polyprion americanus'')]]
[[File:Eptatretus stoutii 1.jpg|thumb|right|[[Pacific hagfish]] trying to hide under a rock]]
Hagfish can exude copious quantities of a milky and fibrous slime or [[mucus]], from specialized slime glands.<ref name=":5"/> When released in seawater, the slime expands to 10,000 times its original size in 0.4 seconds.<ref>{{Cite web |last=Chodosh |first=Sara |date=25 August 2021 |title=Here's how hagfish slime gets 10,000 times bigger in 0.4 seconds |url=https://www.popsci.com/hagfish-slime-expands/ |access-date=29 August 2024 |website=[[Popular Science]] |language=en-US}}</ref> This slime that hagfish excrete has very thin fibers that make it more durable and retentive than the slime excreted by other animals.<ref>{{cite journal |last1=Fudge |first1=Douglas |last2=Levy |first2=Nimrod |last3=Chiu |first3=Scott |last4=Gosline |first4=John |title=Composition, morphology and mechanics of hagfish slime |journal=Journal of Experimental Biology |date=2005 |volume=208 |issue=24 |pages=4613–4625 |doi=10.1242/jeb.01963|pmid=16326943 |s2cid=16606815 |doi-access= }}</ref> The fibers are made of proteins and also make the slime flexible. If they are caught by a predator, they can quickly release a large amount of slime to escape.<ref>{{cite journal |last1=Böni |first1=Lukas |last2=Fischer |first2=Peter |last3=Böcker |first3=Lukas |last4=Kuster |first4=Simon |last5=Rühs |first5=Patrick |title=Hagfish slime and mucin flow properties and their implications for defense |journal=Scientific Reports |date=2016 |volume=6 |page=30371 |doi=10.1038/srep30371|pmid=27460842 |pmc=4961968 |bibcode=2016NatSR...630371B }}</ref> If they remain captured, they can tie themselves in an [[overhand knot]], and work their way from the head to the tail of the animal, scraping off the slime and freeing themselves from their captor. [[Rheology|Rheological]] investigations showed that hagfish slime [[viscosity]] increases in elongational flow which favors gill clogging of [[Aquatic feeding mechanisms|suction feeding fish]], while its viscosity decreases in [[Shear stress|shear]] which facilitates scraping off the slime by the travelling-knot.<ref>{{cite journal |last1=Böni |first1=Lukas |last2=Fischer |first2=Peter |last3=Böcker |first3=Lukas |last4=Kuster |first4=Simon |last5=Rühs |first5=Patrick A. |title=Hagfish slime and mucin flow properties and their implications for defense |journal=Scientific Reports |date=September 2016 |volume=6 |issue=1 |pages=30371 |doi=10.1038/srep30371 |pmid=27460842 |pmc=4961968 |bibcode=2016NatSR...630371B |doi-access=free }}</ref>

Recently, the slime was reported to entrain water in its [[keratin]]-like [[intermediate filament]]s excreted by [[Gland thread cell|gland thread cells]], creating a slow-to-dissipate, viscoelastic substance, rather than a simple gel. It has been shown to impair the function of a predator fish's [[gill]]s. In this case, the hagfish's mucus would clog the predator's gills, disabling their ability to respire. The predator would release the hagfish to avoid suffocation. Because of the mucus, few marine predators target the hagfish. Other predators of hagfish are varieties of birds or mammals.<ref>{{cite journal |doi=10.1242/jeb.02067 |last1=Lim |first1=J |last2=Fudge |first2=DS |last3=Levy |first3=N |last4=Gosline |first4=JM |title=Hagfish slime ecomechanics: testing the gill-clogging hypothesis |date=January 31, 2006 |journal=Journal of Experimental Biology |volume=209|issue=Pt 4 |pages=702–710 |pmid=16449564 |doi-access=free }}</ref>

Free-swimming hagfish also slime when agitated, and later clear the mucus using the same travelling-knot behavior.<ref>{{Cite book |last=Martini |first=F. H. |year=1998 |chapter=The ecology of hagfishes |title=The Biology of Hagfishes |editor-first=J. M. |editor-last=Jørgensen |editor2-first=J. P. |editor2-last=Lomholt |editor3-first=R. E. |editor3-last=Weber |editor4-first=H. |editor4-last=Malte |pages=57–77 |place=London |publisher=Chapman and Hall |isbn=978-0-412-78530-6 |chapter-url=https://books.google.com/books?id=vu1uzL0p7xsC&pg=PA57 }}</ref><ref>{{Cite journal |author=Strahan, R. |year=1963 |title=The behavior of myxinoids |journal=Acta Zoologica |volume=44 |issue=1–2 |pages=73–102 |doi=10.1111/j.1463-6395.1963.tb00402.x}}</ref> The reported gill-clogging effect suggests that the travelling-knot behavior is useful or even necessary to restore the hagfish's own gill function after sliming.

Hagfish thread keratin ([[Pacific hagfish|EsTKα]] and EsTKγ; {{UniProt|Q90501}} and {{UniProt|Q90502}}), the protein that make up its slime filaments, is under investigation as an alternative to [[spider silk]] for use in applications such as body armor.<ref>{{Cite news|url=https://nypost.com/2017/10/25/slime-from-this-300-million-year-old-creature-could-create-bulletproof-body-armor/|title=Slime from this 300 million-year-old creature could create bulletproof body armor|date=2017-10-25|work=New York Post|access-date=2017-10-26|language=en-US|archive-date=2017-10-25|archive-url=https://web.archive.org/web/20171025192741/http://nypost.com/2017/10/25/slime-from-this-300-million-year-old-creature-could-create-bulletproof-body-armor/|url-status=live}}</ref> These [[alpha-keratin]] proteins in hagfish slime transform from an [[alpha helix|α-helical]] structure to a stiffer [[β sheet]] structure when stretched.<ref>{{cite journal |last1=Fu |first1=Jing |last2=Guerette |first2=Paul A. |last3=Miserez |first3=Ali |title=Self-Assembly of Recombinant Hagfish Thread Keratins Amenable to a Strain-Induced α-Helix to β-Sheet Transition |journal=Biomacromolecules |date=8 July 2015 |volume=16 |issue=8 |pages=2327–2339 |doi=10.1021/acs.biomac.5b00552|pmid=26102237 }}</ref> With combined draw-processing (stretching) and chemical crosslinking, [[recombinant DNA|recombinant]] slime keratin turns into a very strong fiber with an [[Young's modulus|elastic modulus]] reaching 20&nbsp;GPa.<ref>{{cite journal |last1=Fu |first1=Jing |last2=Guerette |first2=Paul A. |last3=Pavesi |first3=Andrea |last4=Horbelt |first4=Nils |last5=Lim |first5=Chwee Teck |last6=Harrington |first6=Matthew J. |last7=Miserez |first7=Ali |title=Artificial hagfish protein fibers with ultra-high and tunable stiffness |journal=Nanoscale |date=2017 |volume=9 |issue=35 |pages=12908–12915 |doi=10.1039/c7nr02527k |pmid=28832693}}
*{{cite magazine |author=Harriet Brewerton |date=15 September 2017 |title=Hagfish slime turned into ultra-stiff fibre |magazine=Chemistry World |url=https://www.chemistryworld.com/news/hagfish-slime-turned-into-ultra-stiff-fibre/3007995.article |access-date=1 June 2019 |archive-date=1 June 2019 |archive-url=https://web.archive.org/web/20190601194038/https://www.chemistryworld.com/news/hagfish-slime-turned-into-ultra-stiff-fibre/3007995.article |url-status=live }}</ref>

When in 2017 a road accident on [[U.S. Route 101 in Oregon|U.S. Highway 101]] resulted in {{convert|7500|lb}} of hagfish being spilled, they emitted sufficient slime to cover the road and a nearby car.<ref>{{cite web|url=https://edition.cnn.com/2017/07/14/us/slime-eels-highway-accident-oregon-trnd/index.html|title=Slime eels cause multiple car pileup on Oregon highway|website=CNN.com|last=LeBlanc|first=Paul|date=14 July 2017|access-date=21 April 2022|archive-date=20 June 2022|archive-url=https://web.archive.org/web/20220620103114/https://amp.cnn.com/cnn/2017/07/14/us/slime-eels-highway-accident-oregon-trnd/index.html|url-status=live}}</ref>

===Respiration===
A hagfish generally respires by taking in water through its [[pharynx]], past the velar chamber, and bringing the water through the internal [[gill]] pouches, which can vary in number from five to 16 pairs, depending on species.<ref>{{cite book|author1=Springer, Joseph|author2=Holley, Dennis|title=An Introduction to Zoology|url=https://books.google.com/books?id=BzgNZca_L5AC&pg=PA376|date=2012|publisher=Jones & Bartlett Publishers|isbn=978-1-4496-9544-6|pages=376–|access-date=2016-03-13|archive-date=2024-08-29|archive-url=https://web.archive.org/web/20240829210757/https://books.google.com/books?id=BzgNZca_L5AC&pg=PA376|url-status=live}}</ref> The gill pouches open individually, but in'' Myxine'', the openings have coalesced, with canals running backwards from each opening under the skin, uniting to form a common aperture on the [[ventral]] side known as the branchial opening. The [[esophagus]] is also connected to the left branchial opening, which is therefore larger than the right one, through a pharyngocutaneous duct (esophageocutaneous duct), which has no respiratory tissue. This pharyngocutaneous duct is used to clear large particles from the pharynx, a function also partly taking place through the nasopharyngeal canal. In other species, the coalescence of the gill openings is less complete, and in ''Bdellostoma'', each pouch opens separately to the outside, as in lampreys.<ref>{{cite book|last=Hughes |first=George Morgan |title=Comparative Physiology of Vertebrate Respiration|url=https://archive.org/details/ost-biology-comparativephysi00hugh|date=1963|publisher=Harvard University Press|isbn=978-0-674-15250-2|pages=[https://archive.org/details/ost-biology-comparativephysi00hugh/page/n26 9]–}}</ref><ref>{{cite book|author=Wake, Marvalee H.|title=Hyman's Comparative Vertebrate Anatomy|url=https://books.google.com/books?id=VKlWjdOkiMwC&pg=PA81|date=1992|publisher=University of Chicago Press|isbn=978-0-226-87013-7|pages=81–|access-date=2016-03-13|archive-date=2024-08-29|archive-url=https://web.archive.org/web/20240829210815/https://books.google.com/books?id=VKlWjdOkiMwC&pg=PA81#v=onepage&q&f=false|url-status=live}}</ref> The unidirectional water flow passing the gills is produced by rolling and unrolling velar folds located inside a chamber developed from the nasohypophyseal tract, and is operated by a complex set of muscles inserting into cartilages of the neurocranium, assisted by peristaltic contractions of the gill pouches and their ducts.<ref>{{cite book|author1=Bone, Quentin|author2=Moore, Richard|title=Biology of Fishes|url=https://books.google.com/books?id=e2N4AgAAQBAJ&pg=PA128|date=2008|publisher=Taylor & Francis|isbn=978-1-134-18631-0|pages=128–|access-date=2016-03-13|archive-date=2024-08-29|archive-url=https://web.archive.org/web/20240829210751/https://books.google.com/books?id=e2N4AgAAQBAJ&pg=PA128#v=onepage&q&f=false|url-status=live}}</ref> Hagfish also have a well-developed dermal capillary network that supplies the skin with oxygen when the animal is buried in anoxic mud, as well as a high tolerance for both hypoxia and anoxia, with a well-developed anaerobic metabolism.<ref name="Jørgensen">{{cite book|author=Jørgensen, Jørgen Mørup |title=The Biology of Hagfishes|url=https://books.google.com/books?id=vu1uzL0p7xsC&pg=PA231|year=1998|publisher=Springer Science & Business Media|isbn=978-0-412-78530-6|pages=231–}}</ref> Members of the group have spent 36 hours in water completely devoid of dissolved oxygen, and made a complete recovery.<ref>[https://www.sciencedirect.com/science/article/abs/pii/S1095643321001501 Hypoxia modifies calcium handling in the Pacific hagfish, Eptatretus stoutii]</ref> The skin has also been suggested to be capable of [[cutaneous respiration]].<ref>{{cite book|author1=Helfman, Gene|author2=Collette, Bruce B.|author3=Facey, Douglas E.|author4=Bowen, Brian W.|title=The Diversity of Fishes: Biology, Evolution, and Ecology|url=https://books.google.com/books?id=FyehAR6hsUUC&pg=PA235|date=2009|publisher=John Wiley & Sons|isbn=978-1-4443-1190-7|pages=235–|access-date=2016-03-13|archive-date=2024-08-29|archive-url=https://web.archive.org/web/20240829210753/https://books.google.com/books?id=FyehAR6hsUUC&pg=PA235#v=onepage&q&f=false|url-status=live}}</ref>

===Nervous system===

[[File:Dorsal, left lateral views of dissected hagfish brain.jpg|thumb|left|Dorsal / left lateral views of dissected hagfish brain, scale bar added for size]]

The origins of the vertebrate nervous system are of considerable interest to evolutionary biologists, and cyclostomes (hagfish and lampreys) are an important group for answering this question. The complexity of the hagfish brain has been an issue of debate since the late 19th century, with some morphologists suggesting that they do not possess a [[cerebellum]], while others suggest that it is continuous with the [[midbrain]].<ref>{{Citation| last1 = Larsell | first1 = O | title = The cerebellum of myxinoids and petromyzonts including developmental stages in the lampreys. | journal = Journal of Experimental Biology | volume = 210 | issue = 22 | year = 1947 | pages = 3897–3909 | doi = 10.1002/cne.900860303 | pmid = 20239748 | s2cid = 36764239 }}</ref> It is now considered that the hagfish neuroanatomy is similar to that of lampreys.<ref>{{Citation| last1 = Wicht | first1 = H | title = The brains of lampreys and hagfishes: Characteristics, characters, and comparisons. | journal = Brain, Behavior and Evolution | volume = 48 | issue = 5 | year = 1996 | pages = 248–261 | doi = 10.1159/000113204 | pmid = 8932866 }}</ref> A common feature of both cyclostomes is the absence of [[myelin]] in neurons.<ref>{{Cite journal| last1 = Bullock | first1 = T.H. | last2 = Moore | first2 = J.K. | last3 = Fields | first3 = R.D. | title = Evolution of myelin sheaths: both lamprey and hagfish lack myelin. | journal = Neuroscience Letters | volume = 48 | issue = 2 | year = 1984 | pages = 145–148 | doi = 10.1016/0304-3940(84)90010-7| pmid = 6483278 | s2cid = 46488707 }}</ref> The brain of a hagfish has specific parts similar to the brains of other vertebrates.<ref name=":4">{{cite journal |last1=Ota |first1=Kinya |last2=Kuratani |first2=Shigeru |title=Developmental Biology of Hagfishes, with a Report on Newly Obtained Embryos of the Japanese Inshore Hagfish, Eptatretus burgeri |journal=Zoological Science |date=2008 |volume=25 |issue=10 |pages=999–1011 |doi=10.2108/zsj.25.999 |pmid=19267636 |s2cid=25855686 }}</ref> The dorsal and ventral muscles located towards the side of the hagfish body are connected to [[spinal nerves]]. The spinal nerves that connect to the muscles of the pharyngeal wall grow individually to reach them.<ref>{{cite journal |last1=Oisi |first1=Yasuhiro |last2=Fujimoto |first2=Satoko |last3=Ota |first3=Kinya |last4=Kuratani |first4=Shigeru |title=On the peculiar morphology and development of the hypoglossal, glossopharyngeal and vagus nerves and hypobranchial muscles in the hagfish |journal=Zoological Letters |date=2015 |volume=1 |issue=6 |page=6 |doi=10.1186/s40851-014-0005-9|pmid=26605051 |pmc=4604111 |doi-access=free }}</ref>

===Eye===
The hagfish eye lacks a lens, [[extraocular muscles]], and the three motor cranial nerves (III, IV, and VI) found in more complex vertebrates, which is significant to the study of the [[Evolution of the eye|evolution of more complex eyes]]. A [[parietal eye]] is also absent in extant hagfish.<ref>{{cite book|author=Ostrander, Gary Kent|title=The Laboratory Fish|url=https://books.google.com/books?id=Hp4YSFiSD0IC&pg=PT129|year=2000|publisher=Elsevier|isbn=978-0-12-529650-2|pages=129–|access-date=2016-03-13|archive-date=2024-08-29|archive-url=https://web.archive.org/web/20240829210925/https://books.google.ps/books?id=Hp4YSFiSD0IC&pg=PT129&redir_esc=y#v=onepage&q&f=false|url-status=live}}</ref><ref>{{cite web|url=http://www.physorg.com/news115919015.html|title=Keeping an eye on evolution|date=2007-12-03|access-date=2007-12-04|publisher=PhysOrg.com|archive-date=2012-03-15|archive-url=https://web.archive.org/web/20120315125907/http://www.physorg.com/news115919015.html|url-status=live}}</ref> Hagfish eyespots, when present, can detect light, but as far as it is known, none can resolve detailed images. In ''Myxine'' and ''Neomyxine'', the eyes are partly covered by the trunk musculature.<ref name=r1/> [[Paleontological]] evidence suggests, however, that the hagfish eye is not [[Cladistics#Terminology_for_character_states|plesiomorphic]] but rather degenerative, as fossils from the [[Carboniferous]] have revealed hagfish-like vertebrates with complex eyes. This would suggest that ancestrally Myxini possessed complex eyes.<ref>{{Citation| last1 = Gabbott | first1 = S.E | last2 = Donoghu | first2 = P.C | last3 = Sansom | first3 = R.S | last4 = Vinther | first4 = J | display-authors = 2 | title = Pigmented anatomy in Carboniferous cyclostomes and the evolution of the vertebrate eye. | journal = Proc. R. Soc. B | volume = 283 | issue = 1836 | pages = 20161151 | year = 2016 | doi = 10.1098/rspb.2016.1151 | pmid = 27488650 | pmc = 5013770 }}</ref><ref>{{Citation| last1 = Bardack | first1 = D | title = First fossil hagfish (Myxinoidea): a record from the Pennsylvanian of Illinois | journal = Science | volume = 254 | issue = 5032 | pages = 701–3 | year = 1991 | doi = 10.1126/science.254.5032.701 | pmid = 17774799 | bibcode = 1991Sci...254..701B | s2cid = 43062184 }}</ref>

===Cardiac function, circulation, and fluid balance===
Hagfish are known to have one of the lowest blood pressures among the vertebrates.<ref name="Forster 1985–1992">{{Cite journal|last1=Forster|first1=Malcolm E.|last2=Axelsson|first2=Michael|last3=Farrell|first3=Anthony P.|last4=Nilsson|first4=Stefan|date=1991-07-01|title=Cardiac function and circulation in hagfishes|journal=Canadian Journal of Zoology|volume=69|issue=7|pages=1985–1992|doi=10.1139/z91-277|issn=0008-4301}}</ref> One of the most primitive types of fluid balance found in animals is among these creatures; whenever a rise in extracellular fluid occurs, the blood pressure rises and this, in turn, is sensed by the kidney, which excretes excess fluid.<ref name ="Jørgensen"/> They also have the highest blood volume to body mass of any chordate, with 17 ml of blood per 100 g of mass.<ref>{{Cite web |url=http://cronodon.com/BioTech/hagfish.html |title=Hagfish - Cronodon |access-date=2018-05-04 |archive-date=2018-05-04 |archive-url=https://web.archive.org/web/20180504091106/http://cronodon.com/BioTech/hagfish.html |url-status=live }}</ref>

The hagfish circulatory system has been of considerable interest to evolutionary biologists and present day readers of physiology. Some observers first believed that the hagfish heart was not innervated (as the hearts of jawed vertebrates are),<ref>{{Citation| last1 = Jensen | first1 = D | title = The aneural heart of the hagfish. | journal = Annals of the New York Academy of Sciences | volume = 127 | issue = 1 | pages = 443–58 | year = 1965 | bibcode = 1965NYASA.127..443J | doi = 10.1111/j.1749-6632.1965.tb49418.x | pmid = 5217274 | s2cid = 5646370 }}</ref> but further investigation revealed that the hagfish does have a true innervated heart. The hagfish circulatory system also includes multiple accessory pumps throughout the body, which are considered auxiliary "hearts".<ref name="Forster 1985–1992"/>

Hagfish are the only known vertebrates with osmoregulation isosmotic to their external environment. Their renal function remains poorly described. There is a hypothesis that they excrete ions in bile salts.<ref>{{Citation| last1 = Robertson | first1 = J.D | title = Chemical composition of the body fluids and muscle of the hagfish Myxine glutinosa and the rabbit-fish Chimaera monstros. | journal = Journal of Zoology | volume = 178 | issue = 2 | year = 1976 | pages = 261–277 | doi = 10.1111/j.1469-7998.1976.tb06012.x }}</ref>

===Musculoskeletal system===
Hagfish musculature differs from jawed vertebrates in that they have neither a horizontal septum nor a vertical septum, which in jawed vertebrates are junctions of connective tissue that separate the [[Epaxial and hypaxial muscles|hypaxial musculature and epaxial musculature]]. They do, however, have true [[myomere]]s and myosepta like all vertebrates. The mechanics of their craniofacial muscles in feeding have been investigated, revealing advantages and disadvantages of their dental plate. In particular, hagfish muscles have increased force and gape size compared to similar-sized jawed vertebrates, but lack the speed amplification given by jawed vertebrates' muscles, suggesting that jaws are faster acting than hagfish dental plates.<ref>{{Cite journal| last1 = Clark | first1 = A.J. | last2 = Summers | first2 = A.P. | title = Morphology and kinematics of feeding in hagfish: possible functional advantages of jaws. | journal = Journal of Experimental Biology | volume = 210 | issue = 22 | year = 2007 | pages = 3897–3909 | doi = 10.1242/jeb.006940 | pmid = 17981857 | doi-access =  }}</ref>

[[File:Hagfish trunk.jpg|thumb|left|Vertical section of hagfish midline trunk: The notochord is the only skeletal element, and the musculature has no septum, neither horizontal nor vertical.]]

[[File:Hagfish skull Fig 74 in Kingsley 1912.png|thumb|Hagfish skull Fig 74 in Kingsley 1912]]
The hagfish skeleton comprises the skull, the [[notochord]], and the caudal fin rays. The first diagram of the hagfish endoskeleton was made by Frederick Cole in 1905.<ref>{{Citation| last= Cole | first= F.J. | title = A Monograph on the general Morphology of the Myxinoid Fishes, based on a study of Myxine. Part I. The Anatomy of the Skeleton. | journal = Earth and Environmental Science Transactions of the Royal Society of Edinburgh | volume = 41 | issue = 3 | year = 1906 }}</ref> In Cole's monograph, he described sections of the skeleton that he termed "pseudo-cartilage", referring to its distinct properties compared to jawed chordates. The lingual apparatus of hagfish is composed of a cartilage base bearing two teeth-covered plates (dental plates) articulated with a series of large cartilage shafts<!-- citation -->. The nasal capsule is considerably expanded in hagfish, comprising a fibrous sheath lined with cartilage rings. In contrast to lampreys, the braincase is noncartilaginous. The role of their branchial arches is still highly speculative, as hagfish embryos undergo a caudal shift of the posterior pharyngeal pouches; thus, the branchial arches do not support gills.<ref>{{Cite journal| last1 = Oisi | first1 = Y. | last2 = Fujimoto | first2 = S. | last3 = Ota | first3 = K.G. | last4 = Kuratani | first4 = S. | title = On the peculiar morphology and development of the hypoglossal, glossopharyngeal and vagus nerves and hypobranchial muscles in the hagfish. | journal = Zoological Letters | volume = 1 | issue = 1 | pages = 6 | year = 2015 | doi = 10.1186/s40851-014-0005-9 |doi-access=free | pmid = 26605051 | pmc = 4604111 }}</ref> While parts of the hagfish skull are thought to be homologous with lampreys, they are thought to have very few elements homologous with jawed vertebrates.<ref>{{Cite journal| last1 = Oisi | first1 = Y. | last3 = Fujimoto | first3 = S. | last2 = Ota | first2 = K.G. | last4 = Kuratani | first4 = S. | title = Development of the chondrocranium in hagfishes, with special reference to the early evolution of vertebrates | journal = Zoological Science | volume = 30 | issue = 11 | year = 2013 | pages = 944–961 | doi = 10.2108/zsj.30.944| pmid = 24199860 | s2cid = 6704672 | doi-access = free }}</ref>