supradorsal {neural spine)_£;' 



basidorsal (caudai sclerotomite) 

 neural tube 



sheath of notochord 



perichordol hypocentrum 



aorta 

 caudal vein 

 basiventral (caudal sclerotomite) 



line of myoseptum 



interdorsal (cranial sclerotomite) 



chordal pleurocentrum 



nterventrol (cranial sclerotomite) 



Figure 6-33. Components of a typical vertebra as described for the shark by Goodrich. (After 

 Hymon, 1942) 



Chondrichthyes and agnaths 



Support for the primitive nature of four arcualia per seg- 

 ment comes primarily from the agnaths. In this group, the 

 notochord sheath has an outer elastica externus and a dis- 

 tinct fibrous layer, but it not invaded by cells. Only 

 cartilaginous "neural" arch elements occur, and, except at 

 the anterior end of the column, there are two of these per 

 segment, one in front, and one behind the ventral root of 

 the spinal nerve (Figures 5-25 A, 6-37). The dorsal nerve 

 root lies behind the posterior element. Ventral arch ele- 



ments are found only caudally where a skeletal mass en- 

 closes the end of the notochord and the neural and hemal 

 canals. From this mass, radials extend out to the thin margin 

 of the caudal fin. 



Fossil agnaths lacked ossified vertebral elements, although 

 they may have had cartilaginous ones. 



Shark Support for the four-arcualia concept comes also 

 from the vertebral structure of the Chondrichthyes. In 

 Sqiialus (Figure 6-38), there are two pairs of units above and 

 below the notochord. In the course of development, the 

 anterior dorsal unit, or neural arch, which at first lies in front 



ntervertebrol body 



Figure 6-34. Vertebral structure of Pteroniscu/us. A, thorocic vertebrae in lateral view; B, thoracic 

 vertebra as seen from in front; C, anterior caudal vertebral elements in loteral view. (After Nielsen, 

 1942) 



THE VERTEBRAL COLUMN 



163 



