Fig. 240.—Myxine glutinosa. A, lateral view; B, view of the ventral surface of the head, showing the mouth and tentacles. l.l.p, Lateral pore-like apertures of the mucus-sacs; v, anus.
The family includes a single genus, Myxine, of which the common Hag (M. glutinosa) from the North Atlantic is the best known species (Figs. 92, A, and 240). This Hag-Fish occurs off the coasts of Northern Europe, including the British Isles, as well as on the Atlantic sea-board of North America,[[504]] southwards to Cape Cod. Other species are found off the coasts of Chili and Japan. Myxine is quasi-parasitic in its habits, boring its way into the bodies of large Fishes. By means of its rasping "tongue" it devours all the soft parts of its prey, leaving little more than a mere shell of skin and bones. The Fishes usually attacked are the Cod and other Gadoids, but the Sturgeon is not immune, and the presence of a Hag in the abdominal cavity of a Shark (Lamna cornubica) has been recorded. Myxine has the reputation of being very destructive to Fishes caught on lines, and it is said that whole "catches" have been destroyed by its depredations, so that North Sea fishermen have been forced to change their fishing-ground. To what extent the Hags attack Fishes which are living and free is somewhat uncertain, but the little evidence obtainable seems to point to the conclusion that, as a rule, they only prey on Fishes when the latter are hooked or netted, or injured or dead. When not seeking food the Hag lives in the mud of the sea-bottom at depths ranging to nearly 350 fathoms. They are able to swim very rapidly in an undulatory eel-like fashion. M. glutinosa may grow to a length of nearly two feet. The Hag has been described as a protandrous hermaphrodite, that is, it is first a male and then a female, the gonad of the young first producing spermatozoa, and at a later period becoming an ovary and giving rise to eggs. This view has hitherto met with general acceptance, but it has recently been urged with some force that the presence of the two kinds of sex-cells in a young animal is no proof of functional hermaphroditism, since it is not uncommon "to find immature eggs in the testis of many Vertebrates (Teleosts, Petromyzon, Amphibia), where the assumption of hermaphroditism, to say nothing of its protandric form, is entirely unwarranted."[[505]] Myxine produces eggs similar to those of Bdellostoma. Nothing is known of its breeding habits, or of its embryology.
Fam. 2. Bdellostomatidae.—Gill-sacs 6-14 pairs, all with separate external orifices. Bdellostoma (Fig. 92, B) is found on the Pacific sea-board of both North and South America, at the Cape of Good Hope, and on the coasts of New Zealand. The numerical variation of the gill-sacs in different species, and in different individuals of the same species, and even on opposite sides of the same individual, is very remarkable. Out of 354 examples of the Californian species (B. stouti) examined by Dr. Ayres,[[506]] 101 had 11 gill-sacs on each side; 26 had 11 on one side and 12 on the other; 208 had 12 on each side; 11 had 12 on one side and 13 on the other; and 8 had 13 on each side. Occasional specimens may have 14 gill-sacs on each side. The variations are apparently quite independent of size, age, or sex; and when the gill-sacs are asymmetrically developed, the additional sac may be either on the right side or on the left. In the Chilian species there are 10 gill-sacs on each side, but in the species from the Cape of Good Hope the number is reduced to 6 or 7. Bdellostoma closely resembles Myxine in its habits and mode of feeding. The Californian species attaches itself to the gills or to the isthmus of large Fishes, and then rapidly bores its way into the body, devouring the viscera and muscles but leaving the skin intact. It usually attacks large Flounders and species of Sebastodes, and it is especially destructive to Fishes taken in gill-nets. At Monterey every net in the summer contains the empty shells of eviscerated Fishes, and when these are taken out of the water the Hag scrambles out with great alacrity. Large fishes of even 30 pounds weight are often captured without either flesh or viscera, and it cannot be supposed that they entered the net in this condition.[[507]] The species lives on the sea-bottom most abundantly at a depth of 10-20 fathoms, but becomes rarer as the water deepens or becomes shallower.
Fig. 241.—A, Cluster of the eggs of Bdellostoma stouti, connected by the interlocking of their anchor-shaped filaments; B, the animal pole of an egg, showing the polar "anchors" and the opercular ring. (From Bashford Dean.)
The eggs of the Californian Bdellostoma are large, varying in size from 14.3-29 mm. in length, and from 6.8-10.5 mm. in width, and each egg is enclosed in a horny egg-case secreted by the epithelium of its ovarian ovisac[[508]] (Fig. 241). At each pole of the egg-case there is a tuft of numerous horny filaments which end in 2- 3- or 4-hooked, anchor-like extremities. In the centre of the tuft of filaments at the animal pole of the egg the egg-case is perforated by a micropyle, and a little below this point the case is encircled by an opercular groove, which enables the polar portion to be thrown off like a cap at the time of hatching, so as to allow the young Bdellostoma to make its escape. The large size of the egg, which almost completely fills the cavity of the egg-case, is due to the fact that it consists mainly of food yolk, the germinal protoplasm containing the nucleus forming only a small hillock near the inner extremity of the micropyle. Bdellostoma spawns during the greater part of the year, but chiefly in the early summer, and probably about 20 eggs are deposited at one time, generally on a shelly or rocky bottom. After deposition the eggs become connected together in long chains or clusters by the interlocking of their polar hooks. Fertilisation takes place after extrusion, and the segmentation is meroblastic and discoidal, much as in Teleosts. The embryo completes its development within the egg, and when hatched it is a miniature of the adult.
Fig. 242.—Embryo of Bdellostoma stouti near the time of hatching. (From Bashford Dean.)
Order II. Petromyzontes.