GENERAL CHARACTERS OF MARS1POBRANCH8. 409 



CLASS II. MARSIPOBRA^CHII (Lampreys, or Cydostomi). 



General Characters of the Cyclostomatous Vertebrates. 

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

 less Vertebrates, the body is long and slender, cylindrical, 

 the skin smooth, scaleless, with only a median dorsal and 

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

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

 merous conical teeth. There is no bony skeleton ; the 

 spinal column is represented simply by a thick rod (dorsal 

 cord, notochord) surrounded by a sheath. The skull is car- 

 tilaginous, not movable on the vertebral column ; is very 

 imperfectly developed, having no jaws, the hyo-mandibu- 

 lar 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 developed, the cerebellum in Myxine blended 

 Avitb, 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 

 Marsipobranchii), in the lamprey being seven in number on 

 each side of the pharynx, opening externally by small aper- 

 tures ; internally they connect with a long cavity lying under 

 the oesophagus, and opening anteriorly into the mouth. The 

 heart is like that of fishes, as are the kidneys. The eyes 

 are minute, sunken 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 (pharynx), but 

 in the lamprey forms a cul-de-sac. 



The ovaries and male glands (the sexes being distinct) are 

 impaired plates suspended from the back-bone, and have no 



