This taxon is thought to be the sister group of vertebrates and to be the basal craniate taxon. Extant hagfishes are excluded from the Vertebrata primarily because they lack arcualia (embryonic or rudimentary vertebral elements). This assumes that hagfishes are not degenerate forms of one of the vertebrate groups, and the evidence supports this assumption.
Order MYXINIFORMES (Hyperotreti) (1)—hagfishes. One semicircular canal (and one macula); single olfactory capsule with few folds in sensory epithelium, and olfactory nerves with separate bundles; no bone; lens and extrinsic eye muscles absent; 1-16 pairs of external gill openings; adenohy-pophysis with undifferentiated cellular elements, not divided into distinct regions (unlike in vertebrates); body naked, eel-like; no paired fins; no trace of lateral-line system in adults, neuromasts absent.
Hagfishes are unique among craniates in having only one semicircular canal, which is orientated so that it projects onto all three planes of rotation (lampreys have two and gnathostomes have three) (J0rgensen, 1998; McVean, 1998).
One family (the two subfamilies recognized here are given family status in some works). A probable fossil hagfish, Myxinikela siroka,, of Pennsylvanian age (about 300,000,000 years ago), described in 1991, is known from a single specimen from Illinois (Bardack, 1998). Janvier (1996) speculated that the fossil Gilpichthys, of Mississippian age, might have affinities with the myxiniforms (see also below under Mayomyzontidae).
Family MYXINIDAE (1)—hagfishes. Marine, temperate zones of the world (and Gulfs of Mexico and Panama).
Dorsal fin absent (caudal fin extends onto part of dorsal surface); eyes degenerate; barbels present around biting mouth; teeth only on tongue, plus one on "palate"; dorsal and ventral nerve roots united; nasohypophyseal sac not blind, opening into pharynx; no spiral valve or cilia in intestinal tract; numerous mucous pores along body (shown in sketch); no cerebellum; ovaries and testes in same individual but only one gonad functional; eggs large, yolky, up to 30 per individual; no metamorphosis; low blood pressure. In stating that their eyes are degenerate, it is assumed that hagfishes evolved from an ancestor with eyes, and this is supported by the possible hagfish fossil Myxinikela, which is thought to have had relatively well-developed eyes (Bardack, 1998). There is some variation in the structure of their eyes. In Eptatretus, generally in shallower water than Myxine, the eye has a vitreous body and well-differentiated retina and lies beneath unpigmented skin (presumably the more primitive state), whereas the
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