THE LAMPBEY8. 
141 
Class III. — Marsipobra^^chii {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 slendei-, cylindrical, the 
skin smooth, scaleless, with only a median dorsal and ven- 
tral fin (or in Myxme only a small lower mediaii fin); the 
mouth is circular, and in the lampreys armed with numer- 
ous conical teeth. There is no bony skeleton; the spinal 
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, tlie 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 Marsipohrancliii), 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 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 (pharnyx), but iu 
the lamprey forms a cul-de-sac. 
