Class MYXINI

deepwater Myxine glutinosa lacks a vitreous body, has a poorly differentiated retina, and is buried beneath muscle (Locket and J0rgensen, 1998). The external nasohypophyseal opening is terminal, and it is through this opening that respiratory water passes backward to the gills (unlike lampreys).

Hagfishes are scavenger feeders, mostly eating the insides of dying or dead invertebrates and other fishes. They are the only craniate in which the body fluids are isosmotic with seawater. The mucous pores occur in two ventrolateral lines, each with about 70-200 slime glands that contain mucous cells and thread cells. The thread from the discharged thread cell of hagfishes probably gives tensile strength to the slime. The thread cell itself is not known from any other animals. The secreted slime may be important in feeding and for defense, where it may clog the gills of other fishes and cause suffocation. Hagfishes can go through knotting movements to free themselves from entanglement in slime, escape capture, or tear off food. Extensive information on hagfishes is found in J0rgensen et al. (1998). Maximum length is up to about 1.1 m, attained in Eptatretus carlhubbsi.

Seven genera with about 70 species. The following classification is based largely on Fernholm (1998), except for the recognition of the genera Paramyxine and (Quadratics.

Subfamily Myxininae. Efferent branchial ducts open by a common external aperture on each side (i.e., only one pair of branchial openings). The pharyn-gocutaneous duct, which exits the pharynx behind the gills, is present only on the left side and probably functions to permit the pharynx to be flushed, thus clearing particles too large for the afferent branchial ducts. Four genera and about 25 species.

Myxine. Anal fin ending posterior to branchial aperture; 5 to 7 pairs of gill pouches. Atlantic and Pacific; about 21 species (Wisner and McMillan, 1995, and Fernholm, 1998, recognized 19, but M. limnosa is not recognized here for reasons given in Nelson et al., 2004, to which are added three species from Mincarone, 2001a; Mok and Kuo, 2001; and Mok, 2002).

Notomyxine tridentiger. The pharyngocutaneous duct opens separately to the exterior, leaving two apertures on the left side instead of one as in all other Myxininae (in which it opens into the left common branchial aperture). Buenos Aires to Tierra del Fuego.

Neomyxine biniplicata. A pair of short ventrolateral finfolds behind the branchial region (lateral finfolds are absent in other hagfishes). Cook Strait, New Zealand.

Nemamyxine. Anal fin extending anterior to branchial apertures. Two species, one from New Zealand and the other from southern Brazil, Uruguay, and northern Argentina (Mincarone, 2001b).

Subfamily Eptatretinae. Efferent branchial ducts open separately to the exterior with 5-16 external gill openings.

Three genera, Eptatretus (synonyms Bdellostoma and Polistotrema, 33), Paramyxine (8), and Quadratus (4), with about 45 species (McMillan, 1999; McMillan and Wisner, 2004; Mincarone, 2000; Mincarone and McCosker, 2004; Mok et al., 2001). Fernholm (1998), in recognizing 35 species, treated Paramyxine (with species from Japan and Taiwan) as a synonym of Eptatretus; however, it continues to be recognized by workers such as Mok (2001) and Mok et al. (2001) and is therefore included here. (Quadratus was established for species of Paramyxine with nonlinear and crowded gill apertures by Wisner (1999), who recognized it in its own subfamily, Quadratinae. Wisner (1999) also placed Paramyxine (with gill apertures linear or near linear) in its own subfamily, Paramyxininae (giving three subfamilies rather than the one here). The two new subfamilies were distinguished from the Eptatretinae in having the first efferent branchial duct much longer than the last (versus all being about equal in length). I provisionally recognize the three genera, but place them in the same subfamily because there may be substantial variation in the pattern of the gill apertures (indeed, Fernholm, 1998, preferred regarding Paramyxine as synonymous with Eptatretus because of uncertainty of the validity of this character).

vertebrates. The following taxa, placed within seven superclasses, are recognized in the clade of vertebrates following Donoghue et al. (2000). This monophyletic group, with members possessing or inferred to be derived from ancestors with such features as a dermal skeleton and neural crest, is not formally ranked. However, for the following classification, it could be recognized as the infraclass Vertebrata.

Many of the earliest vertebrate remains are known from isolated microfossils (microvertebrates, ichthyoliths) such as scales and teeth. Their use in providing information on such things as origin, range, and distribution of taxa and for providing phylogenetic characters are reviewed by Turner (2004), particularly for thelodonts and chondrichthyans. In addition to the vast literature on taxa known only from microfossils, Dr. Susan Turner has published many articles in the Newsletter "Ichthyolith Issues."

Anatolepis heintzi—Anatolepis, known from the Upper Cambrian to Lower Ordovician in Spitsbergen and Greenland, was originally described as an agnathan, but its placement as a vertebrate was later questioned. Smith and Sansom (1995), however, showed that dentine is present in the tubercles, and it is placed in the Vertebrata, but of unknown affinities, and not assigned to any higher taxon.

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