GHNERAL CHARACTERS OF MARSIPOBRANCHS, 409 
Crass III. Marsrpoprancuit (Lampreys, or Cyclostomi). 
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 Myzine 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 
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 
Marsipobranchit), 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 esophagus, 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 (Myzine), 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 Myzine communicates with the mouth (pharynx), but 
in the lamprey forms a cul-de-sac. 
The ovaries and male glands (the sexes being distinct) are 
unpaired plates suspended from the back-bone, and have no 
