The mouth of the adult Lamprey is concave, suctorial, fringed by a soft lip bearing numerous short filamentous processes, and armed with numerous epidermic teeth. In the Myxinoidei it has barbules above and below, and the teeth are reduced to a single upper tooth and to two rows borne by the supra-lingual cartilage. The Ammocoetes has a hood-shaped mouth with a fringe of tentacles. The tongue (so called) is of immense size, especially in the Myxinoidei. The mouth (= stomodaeum) is separated from the pharynx by a depending fringed velum in the Myxinoidei. The Ammocoetes has a right and left velar fold; but in the adult a valve with seven projecting filaments overhangs the entrance to the bronchus below the oesophagus (infra) and connects the remnants of the two velar folds which project backwards into it. The alimentary canal is straight. The fore-gut ( = oesophagus and stomach) is narrow, the mid-gut ( = small intestine) begins with a sudden dilatation, into which opens the bile duct; and the hind-gut ( = large intestine and proctodaeum) is very short.

The mid-gut is separated from both fore- and hind-gut by a valve, and in the Lampreys contains a projecting fold or typhlosole, which, beginning on the dorsal side of the intestine, makes a slight spiral twist and ends on the ventral side, and is richly supplied with blood-vessels. The epithelium of the mid-gut is ciliated throughout in Ammocoetes, but in the Lamprey only here and there. The liver consists in the Hags of two separate lobes, each with its own duct and a gall-bladder appended to the point where the ducts fuse and form a common duct. It is united to the ventral abdominal wall in Lampreys, and in the Ammocoetes has a tubular structure with ciliated bile-ducts. At the metamorphosis the tubular structure is lost; fat appears in the cells; the gall-bladder and bile-duct are absorbed. The mid-gut atrophies more or less at the same time. The pancreas is perhaps represented in the Lampreys by an acinous gland opening into the widened commencement of the mid-gut on the left side. The anus lies in front of the urogenital sinus in the adult 1.

The respiratory system consists of gill-pouches or sacs, seven on each side in the Lampreys; six, or very rarely seven, in Myxine; six or seven in Bdellostoma. An eighth, or anterior pouch, is indicated in the embryo Lamprey, but disappears. It corresponds probably with the spiracular cleft of Elasmobranchii. In the Myxinoidei and the Ammocoetes these pouches open internally into the oesophagus; in the Petromyzon into a tube or bronchus underlying the fore-part of the oesophagus, and ending blindly behind. This bronchus is said to be the oesophagus of the Ammocoetes; the oesophagus of the adult being a new structure which grows forwards from behind at the metamorphosis, and opens into the pharynx above the bronchus. Each branchial pouch has a separate external opening in the Lampreys and Bdellostoma, and the separate openings in the Ammocoetes are united by a deep furrow. In Myxine there are only two ventrally-placed external openings, with which the pouches communicate by separate tubes. A ductus oesophageo-cutaneus, or tube, arising from the oesophagus behind the last pouch on the left side, opens externally with the left common aperture in Myxine, with the last pouch of the left side in Bdellostoma. It is not found in Lampreys. The inner surfaces of the pouches carry a series of longitudinal vascular folds, and in the Lampreys tracts of the branchial epithelium are ciliated.

1 The anus represents the blastopore in P. Planeri (Shipley, P. R. S. xxxix. 1885).

The heart has the usual piscine structure, a sinus venosus, auricle and ventricle. The bulbus aortae is small, the ventral aorta long, and gives off as many branchial arteries as there are branchial sacs. Its anterior part is deeply cleft, so that the three or four anterior branchial arteries of either side come off from a right and left common vessel. The sub-intestinal vein of the embryo persists in the adult, and is distributed to the liver forming the portal vein. Its anterior part is said to be pulsatile in Myxine. The blood corpuscles of the Lamprey are circular and nucleated.

The segmental duct persists, and is not divided into Miillerian and Wolffian ducts. It has a coelomic opening in the embryo Ammocoetes, afterwards closed or lost. The kidney in Ammocoetes consists at first solely of a pronephros with funnels opening into the pericardial region of the coelome. This pronephros is afterwards aborted, whilst the anterior part of the mesonephros becomes functional. It is said to be aborted in its turn at the metamorphosis, a posterior series of segmental tubes being developed at the same time. The pronephros of Myxine persists, and lies in the pericardial cavity into which its tubes open. It is cut off, together with the anterior part of the segmental duct, from the mesonephros. This latter consists of the remainder of the segmental duct and a series of short simple segmental tubes, one to each segment. The segmental ducts of the adult open into a urogenital sinus, into which the coelome also opens by a pair of abdominal pores. The sinus in the Lamprey communicates with the exterior by an aperture placed on the apex of a papilla. The papilla lies behind the rectal opening, and is inclosed together with it by a right and left fold of the skin.

The arrangement is essentially the same in Myxine (Ewart). In the Ammocoetes the segmental ducts open into the rectum, but the separation of the urogenital sinus from the rectum, and formation of abdominal pores take place just before metamorphosis. The male and female genital glands are unpaired. In Myxine the suspensory duplicature of peritoneum is broad. The glands in Petromyzon are lobed. Both ova and spermatozoa are shed into the coelome, and pass outwards through the abdominal pores and urogenital sinus. The ova are impregnated externally to the body. The ovum of Petromyzon Planeri is invested by an adhesive mucous coat and a membrane composed of an inner perforated, and an outer structureless layer, and there is a micropyle. Segmentation is total but unequal; and the archenteron is formed by invagination. The epiblast cells grow over the larger yolk cells. The ovum of Myxine is inclosed in an elliptical horny case with processes at each pole. Each process ends in a three-armed anchor, by which the ova adhere one to another in strings, and are probably attached to sea-weed.

The Myxinoidei afford the only instance of parasitism among Vertebrata. They eat their way into the coelome of other Fish, e. g. Cod, Sturgeon, etc. The Lampreys attach themselves to Fish by the suctorial mouth, and scrape their flesh away with the teeth of their tongue.

The Cyclostomi are divided into two Orders as follows: 1. Hyperoariia. Dorsal fin well developed; nasal passage closed. One family, Petromyzontidae, the Lampreys, both marine and fluviatile, with several genera found in various parts of the world; Petromyzon, in Europe, America, Japan, and West Africa; Mordacia, in Tasmania and Chili; Geotria, in South Australia and Chili; Ichthyomyzon, west coast of North America.

2. Hyperotreti. Dorsal fin feebly developed; nasal passage opening into the pharynx. One family, Myxinoidei, containing two genera, Myxine, the Hag-fish, from the northern seas and Pacific coasts of temperate South America; and Bdellostoma, from the southern seas (Cape of Good Hope, New Zealand, coasts of Chili). Exclusively marine.

Anatomy of Petromyzon Planeri, Langerhans, Untersuchungen uber P. Planeri, Freiburg, 1873. Metamorphosis, etc, Schneider Beitrage zur Vergleich. Anat. etc, der Wirbelthiere, Berlin, 1879. Development, Balfour, Comparative Embryology, ii. 1881; of visceral arches, Dohrn, Mitth. Zool. Stat. Naples, v. 1884. Myxinoidei. Vergleich. Anat. des Myxinoiden, J. Muller, Berlin, 1835-40-41; cf. Abhandl. Akad. Berlin, Classis Phyica. 1834,-36,-38,-39. Epidermis of Petromyzon, Foett-inger, Bull. Acad. Roy. Belg. (2) 41. 1876; summary of do., and on Myxine, Blom-field, Q. J. M. xxii. 1882.

Cranium, W. K. Parker, Ph. Tr. 174. 1883; cf. Huxley, Journal of Anat. and Physiol, x. 1876. Skeletal tissue, Gegenbaur, J. Z. v. 1870. Head Muscles, Fur-bringer, J. Z. ix. 1875.

Brain in Ammocoetes and Petromyzon Planeri, Wiedersheim, J. Z. xiv. 1880; in Petromyzon, Ahlborn, Z. W. Z. xxxix. 1883.

Cranial nerves of Petromyzon, Ahlborn, Z. W. Z. xl. 1884. Spinal and visceral nerves of Cyclostomi, Ransom and D'Arcy Thomson, Z. A. ix. 1886.

Pronephros of Myxine, Weldon, Q. J. M. xxiv. 1884; sexual products, Cunningham, Q. J. M. xxvii. (1), 1886. Urogenital system of Cyclostomi, W. Muller, J. Z. ix. 1875.

Abdominal pores and urogenital sinus, Ewart, Journal of Anat and Physiol. x. 1876.