Editing Craniate (section)

==Systematics and taxonomy==
[[File:Figure 29 01 05.jpg|thumb|300px| Craniata, including this [[Placodermi|placoderm]] fish (''[[Dunkleosteus]]'' sp.), are characterized by the presence of a cranium, mandible, and other facial bones.<ref>[https://cnx.org/contents/VB2yhrAh@8/Chordates Chordates] ''OpenStax'', 9 May 2019.</ref> ]]

[[Carl Linnaeus|Linnaeus]] (1758) classified hagfishes as [[Vermes]], a [[Class (biology)|class]] for non-arthropod invertebrates (in modern nomenclature).<ref>{{Cite book |last=Linnaeus |first=Carolus |title=Systema Naturae |year=1758 |edition=10 |language=la}}</ref>

[[André Marie Constant Duméril|Dumeril]] (1806)<ref name="Janvier">{{Cite web |last=Janvier |first=Philippe |title=Craniata - Animals with skulls |url=http://tolweb.org/Craniata |website=Tree of Life Web Project (ToL) |publisher=Tree of Life Web Project}}</ref> grouped hagfishes and lampreys in the taxon Cyclostomi, characterized by horny teeth borne on a tongue-like apparatus, a large notochord as adults, and pouch-shaped gills (Marspibranchii). [[Cyclostomata|Cyclostome]]s were regarded as either degenerate cartilaginous fishes or primitive vertebrates. [[Edward Drinker Cope|Cope]] (1889)<ref name="Janvier"/> coined the name [[Agnatha]] ("jawless") for a group that included the cyclostomes and a number of fossil groups in which jaws could not be observed. Vertebrates were subsequently divided into two major sister-groups: the Agnatha and the Gnathostomata (jawed vertebrates). [[Erik Stensiö|Stensiö]] (1927)<ref name="Janvier"/> suggested that the two groups of living agnathans (i.e. the cyclostomes) arose independently from different groups of fossil agnathans.

[[Søren Løvtrup|Løvtrup]] (1977)<ref name="Janvier"/> argued that lampreys are more closely related to gnathostomes based on a number of uniquely derived characters, including:

*Arcualia (serially arranged paired cartilages above the notochord)
*Extrinsic eyeball muscles
*Radial muscles in the fins
*A closely set atrium and ventricle of the [[heart]]
*Nervous regulation of the heart by the [[vagus nerve]]
*A [[typhlosole]] (a spirally coiled valve of the intestinal wall)
*True [[lymphocyte]]s
*A differentiated anterior lobe of the pituitary gland ([[adenohypophysis]])
*Three inner ear maculae (patches of acceleration sensitive 'hair cells' used in balance) organized into two or three vertical [[semicircular canals]] 
*[[Neuromast]] organs (composed of vibration sensitive hair cells) in the laterosensory canals
*An electroreceptive [[lateral line]] (with voltage sensitive hair cells)
*Electrosensory lateral line nerves
*A [[cerebellum]], i.e. the multi-layered roof of the hindbrain with unique structure (characteristic neural architecture including direct inputs from the lateral line and large output Purkinje cells) and function (integrating sensory perception and coordinating motor control)

In other words, the cyclostome characteristics (e.g. horny teeth on a "tongue", gill pouches) are either instances of convergent evolution for feeding and gill ventilation in animals with an eel-like body shape, or represent primitive craniate characteristics subsequently lost or modified in gnathostomes. On this basis [[Philippe Janvier|Janvier]] (1978){{Citation needed|date=August 2024}} proposed to use the names ''Vertebrata'' and ''Craniata'' as two distinct and nested taxa.