hagfish, primitive marine fish of the order Cyclostomata, or jawless fishes (see cyclostome), of worldwide distribution in cold and temperate waters. Its rudimentary skeleton, of cartilage rather than bone, has a braincase, but no jaw. The circular sucking mouth has rows of horny teeth. There is a single median nostril and the eyes are poorly developed. Like the other jawless fishes, the lampreys, hagfish retain the notochord, a supporting structure found in higher vertebrates only in the embryo, throughout life. They lack a sympathetic nervous system, a spleen, and scales. Hagfish, or hags, spend much time embedded in muddy bottoms. They are chiefly scavengers, but also parasitize slow-moving fishes, eating their way into the victim's body and leaving only the skin and skeleton. Also known as slime eels, hagfish have glands on either side of their bodies that produce enormous quantities of mucoid material, probably as a defense mechanism. The sexes are separate, although an individual may have rudimentary organs of the opposite sex. Spawning occurs throughout the year; no larval stage is known. There are 3 genera and about 20 species of hagfishes. The Atlantic hagfish, Myxine glutinosa, may reach a length of 30 in. (76 cm). The Pacific hagfish, Eptatretus stouti, has been extensively used in physiological studies. The hagfish is classified in the phylum Chordata, subphylum Vertebrata, class Agnatha, order Cyclostomata, family Myxinidae.