CHAPTEE XXIII. 

 THE PURSE-GILLED VERTEBRATES. 



THE next step above the lancelet is the hag-fish. This 

 creature was also once mistaken for a worm. It lives, cov- 

 ered with slime, in the mud deep in the sea, and often bores 

 into the bodies of fishes, living parasitically in them. It is 

 about a foot long, nearly round, with a small mouth. It 

 has no backbone, only a gristly rod, or notocord, extend- 

 ing from head to tail ; it has no bones in its body, no jaws, 

 and only three conical rudimentary teeth within its mouth. 



FIG. 150. Hag-fish or Myxine. ( l /i natural size.) 



The small eyes are hidden beneath the skin. There is but 

 a single opening from the nasal cavity into the mouth ; and 

 while in the true hag-fish there is but a single gill-opening 

 on the under side of the body (p), in a second kind 

 (Bdellostoma) there are seven gill-openings on each side. 

 The creature has only a caudal fin, and its life and in- 

 stincts are not above the level of those of the lower sea- 

 worms. In its brain, however, which is much like that 

 of fishes, the hag-fish shows a promise of better things. 

 The brain is there, but the brain-power is almost dor- 

 mant. 



The lamprey is a step upwards, though but a slight im- 

 provement on the hag-fish. Its large circular mouth, by 

 which it adheres to the body of fishes, is armed within with 



