THE LAMPREYS. 141 



Class III. — Maesipobeanchii {Lampreys, or Cyclostomi). 



General Characters of the Cyclostomatous Vertebrates. — 



In the hag-fish and lamprey, representatives of the jawless 

 Vertebrates, the body is long and sleudei', cylindrical, the 

 skin smooth, scaleless, with only a median dorsal and ven- 

 tral fin (or in Myxine only a small lower median fin); the 

 mouth is circular, and in the lampreys armed with numer- 

 ous conical teeth. There is no bony skeleton; the spina), 

 column is represented simply by a thick rod (dorsal cord, ' 

 notocord) surrounded by a sheath. The skull is cartilag- 

 inous, not movable on the vertebral column; is very imper- 

 fectly developed, having no jaws, the hyo-mandibular bones 

 and the hyoid arch existing in a very rudimentary state. 

 The few teeth present in the hag-fish are confined to the 

 palate and tongue; those of the lamprey are numerous, 

 conical, and developed on the cartilages supporting the lips. 



The nervous system is much as in the fishes, the brain 

 with its olfactory, cerebral lobes, thalami, optic lobes, and 

 medulla being develoj^ed, the cerebellum in Myxine blended 

 with, in the lamprey free from the medulla. The digestive 

 canal is straight, with no genuine stomach, but the liver is 

 much as in higher Vertebrates. The respiratory organs 

 are very peculiar, being purse-like cavities (whence the 

 name MarsipohrancMi), in the lamprey seven in num- 

 ber on each side of the pharynx, opening externally by 

 small apertures; internally they connect with a long cav- 

 ity lying under the cesophagus, and opening anteriorly 

 into the mouth. The heart is like that of fishes, as are 

 the kidneys. The eyes are minute, suniten in the head and 

 under the skin in the hag {Myxine), but larger in the 

 lamprey. 



Another extraordinary feature in the class is the single 

 nasal aperture, as opposed to the two occurring in all higher 

 Vertebrates. The aperture leads to a sac, which in the 

 Myxine communicates with the mouth (pharnyx), but in 

 the lamprey forms a cul-de-sac. 



