Body eel-shaped, naked. Subject to a metamorphosis; in the perfect stage with a suctorial mouth armed with teeth, simple or multicuspid, horny, sitting on a soft papilla. Maxillary, mandibulary, lingual, and suctorial teeth may be distinguished. Eyes present (in mature animals). External nasal aperture in the middle of the upper side of the head. The nasal duct terminates without perforating the palate. Seven branchial sacs and apertures on each side behind the head; the inner branchial ducts terminate in a separate common tube. Intestine with a spiral valve. Eggs small. The larvæ without teeth, and with a single continuous vertical fin.
“Lampreys” are found in the rivers and on the coasts of the temperate regions of the northern and southern hemispheres. Their habits are but incompletely known, but so much is certain that at least some of them ascend rivers periodically, for the purpose of spawning, and that the young pass several years in rivers, whilst they undergo a metamorphosis (see p. 170). They feed on other fishes, to which they suck themselves fast, scraping off the flesh with their teeth. Whilst thus engaged they are carried about by their victim; Salmon have been captured in the middle course of the Rhine with the Marine Lamprey attached to them.
Fig. 315.—Mouth of Larva of Petromyzon branchialis.
Fig. 316.—Mouth of Petromyzon fluviatilis. mx, Maxillary tooth; md, Mandibulary tooth; l, Lingual tooth; s, Suctorial teeth.
Petromyzon.—Dorsal fins two, the posterior continuous with the caudal. The maxillary dentition consists of two teeth placed close together, or of a transverse bicuspid ridge; lingual teeth serrated.
The Lampreys belonging to this genus are found in the northern hemisphere only; the British species are the Sea-Lamprey (P. marinus), exceeding a length of three feet, and not uncommon on the European and North American coasts; the River-Lamprey or Lampern (P. fluviatilis), ascending in large numbers the rivers of Europe, North America, and Japan, and scarcely attaining a length of two feet; the “Pride” or “Sand-Piper” or Small Lampern (P. branchialis), scarcely twelve inches long, the larva of which has been long known under the name of Ammocoetes.
Ichthyomyzon from the western coasts of North America is said to have a tricuspid maxillary tooth.
Mordacia.—Dorsal fins two, the posterior continuous with the caudal. The maxillary dentition consists of two triangular groups, each with three conical acute cusps; two pairs of serrated lingual teeth.