The Hyperotreta, or Hagfishes.—The Hyperotreta (ὑπερῴα, palate; τρετός, perforate), or hagfishes, have the nostril highly developed, a tube-like cylinder with cartilaginous rings penetrating the palate. In these the eyes are little developed and the species are parasitic on other fishes. In Polistotrema stouti, the hagfish of the coast of California, is parasitic on large fishes, rockfishes, or flounders. It usually fastens itself at the throat or isthmus of its host and sometimes at the eyes. Thence it works very rapidly to the inside of the body. It there devours all the muscular part of the fish without breaking the skin or the peritoneum, leaving the fish a living hulk of head, skin, and bones. It is especially destructive to fishes taken in gill-nets. The voracity of the Chilean species Polistotrema dombeyi is equally remarkable. Dr. Federico T. Delfin finds that in seven hours a hagfish of this species will devour eighteen times its own weight of fish-flesh. The intestinal canal is a simple tube, through which most of the food passes undigested. The eggs are large, each in a yellowish horny case, at one end of which are barbed threads by which they cling together and to kelp or other objects. In the California hagfish, Polistotrema stouti, great numbers of these eggs have been found in the stomachs of the males.
Fig. 291.—California Hagfish, Polistotrema stouti Lockington.
Similar habits are possessed by all the species in the two families, Myxinidæ and Eptatretidæ. In the Myxinidæ the gill-openings are apparently single on each side, the six gills being internal and leading by six separate ducts to each of the six branchial sacs. The skin is excessively slimy, the extensible tongue is armed with two cone-like series of strong teeth. About the mouth are eight barbels.
Of Myxine, numerous species are known—Myxine glutinosa, in the north of Europe; Myxine limosa, of the West Atlantic; Myxine australis, and several others about Cape Horn, and Myxine garmani in Japan. All live in deep waters and none have been fully studied. It has been claimed that the hagfish is male when young, many individuals gradually changing to female, but this conclusion lacks verification and is doubtless without foundation.
In the Eptatretidæ the gill-openings, six to fourteen in number, are externally separate, each with its own branchial sac as in the lampreys.
The species of the genus Eptatretus (Bdellostoma, Heptatrema, and Homea, all later names for the same group) are found only in the Pacific, in California, Chile, Patagonia, South Africa, and Japan. In general appearance and habits these agree with the species of Myxine. The species with ten to fourteen gill-openings (dombeyi: stouti) are sometimes set off as a distinct genus (Polistotrema), but in other regards the species differ little, and frequent individual variations occur. Eptatretus burgeri is found in Japan and Eptatretus forsteri in Australia.
The Hyperoartia, or Lampreys.—In the order Hyperoartia, or lampreys, the single nostril is a blind sac which does not penetrate the palate. The seven gill-openings lead each to a separate sac, the skin is not especially covered with mucus, the eyes are well developed in the adult, and the mouth is a round disk armed with rasp-like teeth, the comb-like teeth on the tongue being less developed than in the hagfishes. The intestine in the lampreys has a spiral valve. The eggs are small and are usually laid in brooks away from the sea, and in most cases the adult lamprey dies after spawning. According to Thoreau, "it is thought by fishermen that they never return, but waste away and die, clinging to rocks and stumps of trees for an indefinite period, a tragic feature in the scenery of the river-bottoms worthy to be remembered with Shakespeare's description of the sea-floor." This account is not far from the truth, as recent studies have shown.
The lampreys of the northern regions constitute the family of Petromyzonidæ. The larger species (Petromyzon, Entosphenus) live in the sea, ascending rivers to spawn, and often becoming land-locked and reduced in size by living in rivers only. Such land-locked marine lampreys (Petromyzon marinus unicolor) breed in Cayuga Lake and other lakes in New York. The marine forms reach a length of three feet. Smaller lampreys of other genera six inches to eighteen inches in length remain all their lives in the rivers, ascending the little brooks in the spring, clinging to stones and clods of earth till their eggs are deposited. These are found throughout northern Europe, northern Asia, and the colder parts of North America, belonging to the genera Lampetra and Ichthyomyzon. Other and more aberrant genera from Chile and Australia are Geotria and Mordacia, the latter forming a distinct family, Mordaciidæ. In Geotria, a large and peculiar gular pouch is developed at the throat. In Macrophthalmia chilensis from Chile the eyes are large and conspicuous.