LAMPREYS. 



from Fishes to Man, is wholly wanting in 

 the Eound-mouths. They have, indeed, a 

 supei-ficial, cartilaginous gill-skeleton, but this 

 is of quite different morphological significance. 

 On the other hand, in them we meet, for the 

 first time, with a brain, that important 

 mental organ, which has been transmitted 

 from the Smgle-nostrils up to Man. It is true 

 that in the Round-mouths the brain appears 

 merely as a very small and comparatively 

 insignificant swelling of the spinal chord ; at 

 first a simple bladder (Plate XL Fig. 16, m^), 

 which afterwards separates into five consecu- 

 tive brain- bladders, as in the brains of all 

 Double -breathers. These five simple primitive 

 brain-bladders, which reappear in a similar 

 form in the embryos of all higher Vertebrates, 

 from Fishes up to Man, and which undergo 

 a very complex modification, remain in the 

 Roimd-mouths, in a very low and undifferen- 

 tiated stage of development. The histological 

 elementary structure of the nervous system is 

 also much more imperfect than in other Verte- 

 brates. While in the latter the organ ot 

 hearing always has three semi-circular canals, 

 in the Lampreys it has but two, and in the 

 Hags but one. In most other points also, 

 the organization of the Round-mouths is 



Fig. 190. — The large Sea-lamprey (Petromyzon mari- 

 mis), much redaced in size. A series of seven gill-open- 

 ings are visible below the eye. 



28 



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