Hagfish are one of two groups of jawless fish (the other being lampreys), which are the most primitive true vertebrates. They are members of the Petromyzontidae family. Fishlike vertebrates, jawless fish are similar to eels in form, with a cartilaginous or fibrous skeleton that has no bones. They have no paired limbs and no developed jaws or bony teeth. Their extremely slimy skin lacks scales.

The repulsive-looking hag is the most primitive of all living fish, resembling an outsize, slimy worm. The hag is exclusively marine, and only one family, Myxinidae, is known. The hag has the ability to discharge slime from its mucous sacs, which are far out of proportion to its size.

Their habit of feeding primarily on dead or disabled fish makes hagfish doubly unattractive. Commercial fishermen consider them a great nuisance because they penetrate the bodies of hooked or gillnetted fish, eating out first the intestines and then the meat, leaving nothing but skin and bones.

The hagfish bores into the cavity of its victim by means of a rasplike tongue. Unlike many lampreys (see), it is not a parasite. Hags’ eyes are not visible externally, and they are considered blind. Food is apparently detected by scent, and large numbers of hags are often taken in deepset eel pots baited with dead fish.

The hag can be differentiated from its close relative the lamprey, by the following characteristics: The hag has prominent barbels on its snout, no separate dorsal fin, eyes that are not visible externally, a nasal opening at the tip of the snout, and a mouth that is not funnelshaped or disklike. The largest hags are 2 feet or more in length. They range the cold, deep waters, and at least one specimen was recorded at a depth of 4,380 feet.