[Bigelow and Schroeder, 1948 p. 34.]
The hag, like the lamprey, lacks paired fins and fin rays. Its skeleton is wholly cartilaginous, without bones, its mouth is jawless; and its skin is scaleless. It is easily recognized by its eel-like form; by its single finfold (a fold of skin, not a true fin) running right around the tail and forward on the lower surface of the body with no division into dorsal, caudal, and anal fins; by the single gill pore on each side, just forward of the origin of the ventral finfold; by its lipless mouth, star-shaped in outline when closed; by the single nasal aperture at the tip of the snout; by its peculiar barbels or "tentacles," two flanking the mouth on either side and four surrounding the nostril; and by the evertible tongue studded with rows of horny rasplike "teeth." We might also mention the series of mucous sacs on either side of the abdomen, and point out that the dorsal finfold originates about two-thirds of the distance back from snout toward tip of tail, and the ventral fin fold one-third the way back, with the vent piercing it.
Hags vary in color, perhaps to correspond with the color of the bottom, being grayish brown or reddish gray above, variously suffused, mottled, or piebald with darker or paler gray, with brown, or with bluish; they are whitish or pale gray below.
Gulf of Maine hags grow commonly to a length of about 1½ to 2 feet, with a maximum of 31 inches recorded off the coast of Maine.
The hag is found chiefly if not exclusively where the bottom is soft mud, where (to judge from its actions during the brief time it survives in aquaria) it spends its time lying embedded in the clay or mud with the tip of the snout projecting. And it is at home only in comparatively low temperatures, cooler probably, than 50°, which confines it in summer to depths of 15 to 20 fathoms or more in the Gulf of Maine. It is not a true parasite, as has sometimes been suggested, their being no reason to believe it ever attacks living, uninjured fish, but is a scavenger.[page 11]
Being blind, it doubtless finds its food by its greatly specialized olfactory apparatus. It feeds chiefly on fish, dead or disabled, though no doubt any other carrion would serve it equally well. And it is known to prey on marine annelid worms also, at least in Norwegian waters. It is best known for its troublesome habit of boring into the body cavities of hooked or gilled fishes, eating out the intestines first and then the meat, and leaving nothing but a bag of skin and bones, inside of which the hag itself is often hauled aboard, or clinging to the sides of a fish it has just attacked. In fact, it is only in this way, or entangled on lines, that hags ordinarily are taken or seen.
Being worthless itself, the hag is an unmitigated nuisance, and a particularly loathsome one owing to its habit of pouring out slime from its mucous sacs in quantity out of all proportion to its small size. One hag, it is said, can easily fill a 2-gallon bucket, nor do we think this any exaggeration.
In American waters the commercial fishes most often damaged by it are haddock and the hakes (Urophycis), these being the species most often fished for with long lines or with gill nets over the type of bottom the hag frequents. But it sometimes damages cod also, and European authors describe it as attacking ling (Molva) and other members of the cod tribe, herring, mackerel, sturgeon, and even mackerel sharks under similar circumstances.
The hag was formerly believed to be a functional hermaphrodite, with its single sex organ first developing sperm in the posterior portion, eggs later in the anterior portion. However, recent detailed studies of the sex organ appear to show that such is not the case, but that either the male portion of the common sex organ matures in a given individual with the female portion remaining rudimentary, or vice versa.
It has long been known that the eggs are few in number (only 19 to 30 having been counted in any one female) and large (up to 25 mm. in length), and the horny shell has a cluster of anchor-tipped filaments at each end that make the eggs easy of identification. Until 1900 none had been found that certainly had been laid naturally. In that year, however, hag eggs were reported from the western part of Georges Bank and from the south coast of Newfoundland by Dean (1900); from the neighborhood of the Faroe Islands by Jensen; from Norway by Hjort; off Morocco by Koefoed. And they have been reported subsequently from the Bay of Fundy by Huntsman, from Frenchman Bay on the coast of Maine by Conel. The eggs are deposited on bottom, where they stick firmly to fixed objects of one sort or another by their terminal filaments and by threads of slime.
The hag spawns throughout its range; also it spawns throughout the year, for females nearing ripeness and others nearly spent have been recorded for winter and spring, as well as summer and autumn, in one part of its range or another. The few eggs so far reported have been from depths of 50 to 150 fathoms, most of them trawled on mud, clay, or sand bottom.
We need only add that, to judge from their behavior in aquaria, the females cease to feed at the approach of sexual maturity, as many other fishes do. Newly hatched hags have never been seen, but inasmuch as the smallest yet described (about 2½ inches long), probably not long out of the egg, already resembled the adult in external appearance there is no reason to suppose that the hag passes through a larval stage greatly different from the adult.
Arctic seas, and both coasts of the north Atlantic; Murman Coast and northern Norway south regularly to the Irish Sea, and to Morocco as a stray in the East; northern part of Davis Strait, south to the latitude of Cape Fear, N. C., in the west. It is represented in the corresponding temperature-belt of the Southern Hemisphere by a form (or forms) resembling it so closely that it is doubtful whether any sharp line can be drawn between them.
Apart from one record for the northern part of Davis Strait, the most northerly reports of the hag off the American coast are from southern Newfoundland and from the Grand Banks. But it is generally distributed along outer Nova Scotia at appropriate depths. And it is only too common in the Gulf [page 12] of Maine; perhaps it is not absent there from any considerable area of smooth bottom. Thus, it is abundant off the north end of Grand Manan; is reported from Passaamquoddy Bay and from various localities near Eastport; is to be found off-shore on muddy bottom all along the Maine coast; and is caught at times in considerable numbers on the Boon Island-Isles of Shoals fishing grounds and about Jeffreys Ledge, where we found it plentiful enough in the spring of 1913 to have gutted 3 to 5 percent of all the haddock in the gill nets. Fishermen report it as equally numerous in the deeper parts of Massachusetts Bay. On the offshore banks the hag is well known, and it has been trawled at various localities along the outer edge of the Continental Shelf off New England at depths of from 100 to 200 fathoms, and deeper. We ourselves took 11 large ones in one set of a Monaco deep-sea trap in 260 fathoms off Nantucket on July 9, 1908, and it has been taken in from 300 to 500 fathoms off Marthas Vineyard; as deep as 524 fathoms on the southeast slope of Georges Bank.
 See Bigelow and Schroeder, Fishes Western North Atlantic, Pt. 1, ch. 2, 1948, pp. 35-36, for references.
 Mem. N. Y. Acad. Sci., Vol. 2, Pt. 2, Art. 2, 1900.
 Viden. Meddel. Dansk naturhist. Forening, 1900, p. 1.
 Rept. Norweigian Fishery and Mar. Invest., vol. 1, 1900, No. 1, ch. 4. p. 75.
 Rept. Michael Sars North Atlantic Exped., Zool., vol. 4, No. 1, 1927, p. 18.
 Science, N. Ser., vol. 75, 1932, pp. 19-20.
 It has not been reported for certain from West Greenland (so far as we can learn), from the outer coast of Labrador, or within the Gulf of St. Lawrence though it is to be expected in the deeper parts of the latter.