Elongated Eel-like Ichthyopsida, with the mouth not supported by jaws, as in other Vertebrata; with small azygos fins, but devoid of 'paired fins1; no exo-skeleton; with a single nostril, and six or seven pairs of branchial pouches.
The azygos fin is small and confined to the tail in the Myxinoidei, but extends forwards continuously along the back in the larval Lamprey or Ammocoetes, whereas in the adult or Petromyzon the antero-dorsal portion is separated off as a dorsal fin.
The superficial cells of the epidermis in the Lampreys (Petromyzontidae) develope a cuticular border pierced by fine pores. The deep cells are prismatic and stalked. There are also (1) superficial goblet cells; (2) large oblong club cells, with a distinct membrane and lamellated contents, which gradually reach the surface as they increase in size, and there burst; as well as (3) granular cells of unknown function, spherical or ovoid in shape, with a membrane and nucleated granular contents, and fine processes extending towards the corium. In Myxine, the Hag-fish, the superficial cells of the epidermis are all goblet cells, two to three layers deep, which give origin for the most part to the abundant mucus these fishes throw off. There are besides large oblong cells with granular contents, which reach the surface and burst; and spherical 'spider' cells with clear contents, central stellate granular mass, and a distinct membrane. These two kinds of cells appear to correspond respectively with the club and granular cells of the Lamprey. Myxine possesses also on each side of the body a series of pores leading into pit-like glands imbedded in the subcutaneous tissue. They originate as solid ingrowths of epidermis.
Their cells are of two kinds - 'spider' cells and thread cells - the latter oblong or ovoid, with remains of a nucleus at one pole, a central granular core, and contents differentiated into a thread, wound transversely at the surface, longitudinally in the deeper layers. As soon as the cell is mature, the thread begins to unwind. The epidermis of Petromyzon Planeri contains scattered rod cells, terminating in sensory hairs. P. fluviatilis possesses end buds, each containing but a few cells, and on the head elevated upon papillae. Nerve eminences, somewhat sunk in pits, are found in Petromyzon, arranged in an upper and lower lateral line, and the three typical lines can be traced upon the head. Myxine has none of these structures. There is a remarkable fatty subcutaneous tissue. The myomeres of the body retain their primitive arrangement in Lampreys, but in Myxine the ventro-lateral portion is broken up into an outer oblique layer, and into ventral longitudinal but interrupted bundles. The myomeres overlap one another in the Lampreys, and there are numerous longitudinal fibrous septa stretching from one myocomma to the next succeeding.
The compartments thus formed are filled by the muscle plates.
1 Dohrn believes that the two ridges, one on either side the anus in Petromyzon, represent remnants of pelvic fins. See Mitth. Zool. Stat. Naples, vi. 1885.
The cranium is entirely cartilaginous. It has no roof in the Myxi-noidei, and is only partially roofed in the adult Lamprey. It always retains a large basicranial fontanelle. There appear to be well-developed palatine, pterygoid, quadrate, and hyomandibular regions, a hyoid arch, and in Myxinoidei traces of two branchial arches. The Lamprey is said to have distal rudiments of Meckel's arch. A large basi-hyo-branchial bar supports the tongue. It is carried by the hyoid arch, with which it is continuous in Myxinoidei, and by which it is 'clamped' in Petromyzon. In the latter there are five upper labial cartilages, one median and two paired, and an annular cartilage supports the margin of the mouth. It represents the two anterior lower labials seen in the Anuran Tadpole. There are also three minute lower labial cartilages paired right and left. The upper and lower barbules of the Myxinoidei are supported by cartilaginous rods, but there are no labial cartilages. The Lampreys have a complicated cartilaginous framework supporting the branchiae and the heart. It lies superficially, and must be compared with the extra-branchial cartilages of Anuran Tadpoles and Sharks. The notochord is large, with a tough fibrous sheath.
There are small cartilaginous neural arches in the Lamprey, but not in Myxinoidei, and in the former two cartilaginous rods are applied to the ventro-lateral aspect of the notochord in its whole extent. Cartilaginous rays support the azygos fins.
The brain has the usual parts, but in the larval Lamprey the thalami optici and mid-brain are scarcely separated and are elongated, as is also the medulla oblongata. There are two olfactory lobes, and the cerebellum is a simple transverse commissure. The cerebral hemispheres are solid in the Myxinoidei. The spinal cord is flattened dorso-ventrally. The anterior and posterior roots of the spinal nerves alternate with one another, and do not unite in the Lampreys, but they both divide into dorsal and ventral branches. In Bdellostoma and Myxine there are two anterior roots to every posterior root throughout a portion of the cord at least. Their ventral branches unite, and not their dorsal; but the dorsal branches derived from each pair of anterior roots fuse together. There is no sympathetic system, but the intestinal branches of the vagus extend nearly the whole length of the digestive tract. The nostril is single, even in development, though the olfactory lobes are double. It is tubular, and the tube is supported by cartilage, which in the Myxinoidei forms a series of rings. A passage leads from it to the pharynx, which is blind in the Lamprey, but opens in the Myxinoidei in front of the velum. The eye in the latter wants the eye-muscles, the sclerotic, the iris and lens.
In the larval Lamprey (Ammocoetes) it remains beneath the epidermis, derm and subdermic tissues, which represent the cornea; there is no iris, and the lens retains the embryonic structure. At the metamorphosis the eye travels to the surface, and becomes fully developed. The ear in the Hag-fish consists of a vestibule and single semicircular canal. In the Lamprey there are two vertical semicircular canals, with ampullae as usual, and indications of cochlear and saccular outgrowths. There are motile cilia in the vestibule, as in the otolithic vesicles of Mollusca, etc.