Hagfishes are a group of marine, eel-shaped jawless fishes. There are about 20 species of hagfish, referred to four genera: Myxine, Neomyxine, Paramyxine, and Eptatretus. The former two possess a single pair of common external gill openings, a character of the Myxinidae. The latter two have minute, separate gill openings, which is regarded as the primitive condition. In Paramyxine, however, the gill openings are more closely-set than in Eptatretus, and this has been thought to represent an intermediate condition between Eptatretus and Myxinidae. The eyes, although devoid of lens and extrinsic musculature, are larger in Eptatretus and Paramyxine than in Myxine and Neomyxine, where they are partly covered by the trunk musculature.
Among craniates, hagfishes are unique in many respects, and this has long been regarded as a consequence of degeneracy due to presumed parasitic habits. However, if hagfishes may penetrate inside dead fishes to eat the liver (their favourite dish!), they are in no way endoparasites. On the contrary, they can be active predators and prey on small invertebrates at night. There is now a broad consensus over the idea that most of these unique anatomical and physiological features of hagfishes are in fact primitive and approach the condition of the common ancestor to all craniates. These features are for example their body fluid content (more than 10%, whereas it is less than 10% in all other craniates), the low oxygen affinity of their blood cells, their lack of cardiac innervation, their multiple veinous hearts, their lack of sensory-line neuromasts (although they have acoustico-lateral nerve fibres), their comparatively simple pituitary gland, or their lack of muscles in caudal fin web.
Hagfishes have an antitropical distribution, most species living in relatively cold waters of the northern and southern hemispheres. They have practically no osmoregulation and are very sensitive to drops in water salinity. Having little culinary value (except in Japan), some species of hagfishes are, however, now endangered by their exploitation for their skin, which is extremely tough and soft, and is sold worldwide as "eel skin".
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