Editing Hagfish (section)

===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>