This order includes the Lampreys (Petromyzonidae) and the Hag-fishes (Myxinidae), and is defined by the following characters: - The body is cylindrical, worm-like, and destitute of limbs. The skull is cartilaginous, without cranial bones, and having no lower jaw (mandible). The notochord is persistent, and there are either no vertebral centra, or but the most rudimentary traces of them. The heart consists of one auricle and one ventricle, but the branchial artery is not furnished with a bulbus arteriosus. The gills are sac-like, and are not ciliated.
The type of piscine organisation displayed in the Marsipobranchii is of a very low grade, as indicated chiefly by the persistent notochord without vertebral centra, the absence of any traces of limbs, the absence of a mandible, and the structure of the gills.
Both the Lampreys (fig. 260, B) and the Hag-fishes (fig. 260, A) are vermiform, eel-like fishes, which agree in possessing no paired fins to represent the limbs, but in having a median fin running round the hinder extremity of the body. The skeleton remains throughout life in a cartilaginous condition, the chorda dorsalis is persistent, and the only traces of bodies of vertebrae are found in hardly perceptible rings of osseous matter developed in the sheath of the notochord. The neural arches of the vertebrae, enclosing the spinal cord, are only represented by cartilaginous prolongations. There is a partially cartilaginous cranium, which is not, however, movable on the spinal column. The mouth in the Hag-fish (Myxine) is of a very remarkable character, and enables it to lead a very peculiar mode of life. It is usually found, namely, embedded in the interior of some other large fish, into which it has succeeded in penetrating by means of its singular dental apparatus. The mouth (fig. 260, A) is sucker-like, destitute of jaws, but provided with tactile filaments or cirri. In the centre of the palate is fixed a single, large, recurved fang, which is firmly attached to the under surface of the cranium. The sides of this fang are strongly serrated, and it is by means of this that the Hag-fish bores its way into its victim, having previously attached itself by its sucker like mouth, assisted by the action of the piston-like toothed tongue. In the Lampreys the mouth has also the form of a circular cup or sucker, and is also destitute of jaws; but in addition to the palatine fang of the Myxine, the margins of the lips bear a number of horny processes, which are not really true teeth, but are hard structures developed in the labial mucous membrane. The tongue, also, is armed with serrated teeth, and acts as a kind of piston ; so that the Lampreys are in this manner enabled to attach themselves firmly to solid objects. Sometimes the oral cavity is strengthened by a basket-shaped cartilaginous apparatus, and sometimes a similar apparatus supports the gill-sacs. The alimentary canal is simple and straight, the liver not sac-like, but of its ordinary form, and the kidneys distinct and well developed.
Fig. 260. - Morphology of Marsipobranchii. A, Myxine glutinosa, showing the suckerlike mouth, and the two ventral openings (h) by which the water escapes from the gills. B, The River Lamprey or Lampern (Petromyzon fluviatilis), showing the seven branchial apertures on the side of the neck. C, Branchial organs of Myxine; g The gullet laid open, showing the openings (six on each side) by which the water enters the branchial sacs (s); c Canal carrying the water away from the gills, to be discharged by the two ventrally-placed branchial apertures (h, h); i Aperture by which the water is admitted to the gullet and thence to the gills.
The Marsipobranchii are peculiar amongst Vertebrate animals in possessing only one median nasal sac, opening on the exterior of the head by a single unpaired nostril. The Hag-fishes further differ from all the members of the class, except the Mud-fishes (Dipnoi) in the fact that the nasal cavity communicates behind with the pharynx. In the Lampreys, on the other hand, the nasal sac is closed posteriorly.
Another very remarkable point in the Hag-fishes and Lampreys is to be found in the structure of the gills, from which the name of the order is derived. The gills, namely, are in the form of fixed pouches, instead of being free vascular structures contained in a common chamber, opening externally by a gill-slit, as in the typical Bony Fishes. In the Hag-fishes there are six of these branchial sacs on each side of the oesophagus (fig. 260, C). The water is admitted to the gullet (g) by a special aperture situated on the ventral surface, whence it passes into the branchial pouches by six apertures on each side. Having passed over the complicated and highly vascular interior of the branchial sacs, the water escapes by a corresponding series of tubes opening into a common canal (c) on each side, and these canals finally discharge the effete water by two apertures situated on the ventral surface behind the head (h, h). In the Lampreys the gills have the same fixed and pouch-like arrangement, but there are some marked differences from the above. The water is admitted from the gullet to seven branchial pouches on each side, but the mode of admission is by means of two special canals which lie beneath the oesophagus on each side, communicating each by its own aperture with the mouth in front, terminating blindly behind, and sending off a branch to each pouch. The effete water, also, escapes by a special tube to each sac, so that there are seven branchial apertures in the form of slits or holes on the side of the neck (fig. 260, B). The reproductive organs are ductless, and the generative elements are shed into the abdomen, whence they escape by an abdominal pore.
The Lampreys are, some of them, inhabitants of rivers; but the great Sea-lamprey (Petromyzon marinus) only quits the salt water in order to spawn. The mouth in the Petromyzonidae is a circular cartilaginous ring, formed by the amalgamation of the palatine and mandibular arches, and carrying numerous teeth and small tubercles. The tongue is armed with a double series of small teeth, and acts like a piston, enabling the animal to attach itself to stones and rocks. There is no air-bladder. The body is cylindrical, compressed towards the tail, and destitute of scales. The skeleton consists of a series of cartilaginous rings without ribs. The young Petromyzon undergoes a metamorphosis, being so unlike the parent that a new genus (Ammocaetes) was originally founded for its reception.
In the Myxinidae the mouth is circular and membranous, with eight cirri. The palate carries a single fang, and the tongue is armed with a double row of small teeth on each side. There may be seven branchial apertures on each side (Heptatrema), or the branchial pouches open into a common tube on each side, and each of these terminates in a distinct aperture situated under the heart on the lower surface of the body (Myxine or Gas-trobranchus). The Hags pour out so much mucus through the lateral line that they can surround themselves with jelly; hence the name of the common species (Myxine glutinosa). The Glutinous Hag is a native of the North and British seas, and is chiefly found in the interior of the Cod and Haddock (often five or six individuals in one fish).