Vertebrate Zoology - V. M. Konstantinov 2011

Chordates
Jawless vertebrates
Class Cyclostomata - Order Myxiniformes

Hagfishes are the most parasitic of the cyclostomes. They not only attach themselves to their prey by suction, but also burrow into its body through a created opening or via the gill slits, penetrating deep inside. Thus, they act as temporary endoparasites. They are characterized by A number of simplified structural features. The Skin is naked, featuring abundant mucous glands. The number of gill openings ranges from 1 to 15. The eyes are rudimentary; hagfishes are virtually blind and navigate using their Organs of smell and Touch. The Inner ear contains only a single semicircular canal. The external respiratory openings merge into a common canal that opens far down toward the middle of the body.

Unlike lampreys, their development is direct, meaning there is no larval stage. They feed on fish, predominantly those caught in fishing gear, and can be detrimental to fisheries. They are distributed exclusively in seas and oceans, and have been found in the Atlantic, Pacific, and Indian Oceans.

The Atlantic hagfish (Myxine glutinosa), which has been studied most extensively, reaches a length of about 50 cm. It is distributed in the northern Atlantic Ocean and typically inhabits the shelf zone at depths ranging from 20 to 350 m, and rarely down to 1 000 m. Larger sizes (around 1 m) are attained by certain species of bdellostomids, or hagfishes of the family Eptatretidae (genera Bdellostoma or Eptatretus), which inhabit the Indian and Pacific Oceans off the coasts of North and South America, Japan, Africa, and New Zealand. They are notably voracious and cause significant damage to fisheries.