ELEMENTS off COMPARATIVE ZOOLOGY. 



lower, as in the sharks, it can be recognized throughout 

 life. This notochord is enveloped in a membranous noto- 

 chordal sheath, and in this sheath are formed rings of 

 cartilage which give rise to the bodies (centra) of the verte- 

 brae. Between these rings no cartilage is formed and hence 

 the whole column is jointed and flexible. In the sharks 

 these rings and other parts of the skeleton remain carti- 

 laginous; in other vertebrates any or all may be con- 

 verted into bone. In a typical vertebra, for instance, in 

 the tail of a fish (p. 14), outgrowths from the centrum 

 occur above and below, forming two arches. The upper of 



FIG. 54. Different vertebrae and connected structures. A* in tail region 

 of teleost ; B, in body region of teleost ; C, in tail region of salamander ; 

 D, in mammal ; c, centrum ; 7i, haemal arch (rib in JB) ; w, neural arch ; 

 r, rib ; s, sternum ; t, transverse process. 



these (neural arch) encloses the spinal cord, the lower 

 (haemal arch) extends around the blood-vessels of the tail. 

 Farther forward, in the trunk region of the bony fish, the 

 two halves of the haamal arch do not meet below, but form 

 slender threads (ribs) which support the flesh around the 

 viscera. In the forms above the fishes an outgrowth 

 (transverse process) may rise on either side of the vertebral 

 centrum, and the ribs, when they occur, are continuations of 

 these transverse processes, and have nothing to do with the 



