Course of intersomitic septum , 

 Basidorsal- 



Dorsal ligament 



Dorsal root 

 foramen 

 Ventral root 

 foramen 



Intervertebral 

 disc 



TAXONOMY, ANATOMY, EMBRYOLOGY 



Dorsal branch of 

 nal nerve 



Intersegmental vein 



Spinal ganglion 

 Interventral 



Basiventral 



Dorsal rib 



Mixed spinal nerve 



Figure 9. Shark Vertebrae (Squahis acanthias). (From Goodrich, "Studies on the 

 Structure and Development of Vertebrates," 1909, by permission of The Macmillan 

 Co., publishers. ) 



The vertebrae of Osteichthyes (bony fishes) are not substantially dif- 

 ferent from those of the Chondrichthyes, except that they are ossified in 

 whole or in part. In many fishes, two centra are formed for every body 

 segment, an anterior hypoccntnim based upon the anterior ventral pair 

 of arcualia, and a posterior pleiirocentnim based upon the posterior dorsal 

 pair of arcualia. This is exemplified by the Rhipidistia, an extinct group 

 of lung-breathing fishes which is believed to have given rise to the Am- 

 phibia. 



The most primitive fossil amphibians have vertebrae much like those 

 of the Rhipidistia, but they change in two difl:"erent lines of descent ( Fig- 



hypotenlruni 



neural arch 



neural arch 



Figure 10. Derivation of Amphibian and Amniote Centra. A is the primitive am- 

 phibian type. A, B, and C illustrate the transition to the type of vertebrae found in the 

 modern amphibia, while A, D, and E illustrate the transition to the type found in 

 amniotes. (From Hyman, "Comparative Vertebrate Anatomy," 2nd Ed., Uni\ersitv of 

 Chicago Press, 1942. ) 



S3 



