The lamprey eels and the hagfishes show a great advance in the vertebrate series over the forms thus far considered, but are so different from other fishes that they have been placed in a class by themselves (Fig. 16). They are called “cyclostomes” (cyclo, round; stoma, mouth) for the reason that, instead of typical vertebrate jaws, they have round jawless sucker-like mouths by which they attach themselves to the sides of fishes or to other objects. They are more committed to a life of parisitism than any other vertebrate, and when one fastens to a fish, it may rasp a hole with its filelike horny teeth that are attached to its muscular “tongue,” quite through the skin of the unfortunate host whose death eventually results.
Cyclostomes are distinguished chiefly by the absence of certain customary fishlike structures. They are not only jawless, but are also without paired fins, scales, swim bladder, cloaca, oviducts, true mesodermal teeth, vertebral centra, ribs, or bones of any kind. They have only one external nasal opening instead of the pair present in most vertebrates. They breathe by means of internal gills. All are eel-like in shape, but are not to be confused with true eels, which have a bony skeleton including jaws, instead of a skeleton consisting principally of a persistent notochord.
The larval lamprey is so different from the adult that it was formerly assigned to a distinct genus and named Ammocoetes before its whole life history was known. In its early stages an endostyle-like groove is present in the pharynx. Later this groove pinches off and gives rise to a subpharyngeal gland believed by some to be homologous with the thyroid gland of other vertebrates.
Cyclostomes are usually marine in habitat although they frequent fresh waters to breed, and some species are permanent fresh-water inhabitants.
Lampreys scoop out a nest in the sandy bottom of a flowing stream in which to deposit their eggs, meanwhile fastening themselves by means of their suctorial mouths to a stone in order not to be carried down stream. This habit has given rise to their genus name of Petromyzon (petros, rock; myzon, sucker).
Hagfishes are particularly slippery creatures, often producing so much mucus when uncomfortably confined in a bucket of stagnant water that the water is thickened into a gluey mass. Linnaeus describes Myxine gluiinosa, a European hagfish, in compact Latin: “Intrat et devorat pisces; aquam in gluten mutat.”
Lampreys have for a long time been used for food, particularly in Europe. Cicero in one of his orations bewailed the tendency of young spendthrift Romans of his day, who as in every generation were regarded by their elders as going to the dogs, because they spent their time reveling at night and “feasting upon such delicacies as lampreys.”
Only a few genera are reported as belonging to North America. The best known American and European genera are: the hagfishes Myxine (Atlantic) and Bdellostoma (Pacific) ; the lampreys Petromyzon, found in both salt and fresh water, Ichthyomyzon, of the Great Lakes and Mississippi Valley; and the brook lamprey, Lampetra.
Closely related to the Cyclostomes are the small jawless armored fishes (Fig. 17) known collectively as the Ostracoderms (ostraco, shell; derm, skin). Because of the absence of jaws these two groups are sometimes classified together as the Agnatha (a, without; gnath, jaw). In the ostracoderms, long since extinct inhabitants of fresh waters, thick bony plates developed in the connective tissue membranes of the skin. These plates were large and immovably joined to one another in the head region to form a shell-like structure over this part of the fish. On the rest of the body the scales were smaller, thus permitting the freedom of movement necessary for locomotion.