spinal ligament 

 neural canal 



notochord 



postcardinal 



dorsal root ganglion 

 spinal cord 



neural arcuaiio of one somite 



ventral root foramen 



dorsal root foramen 



segmental vein 

 segmental artery 



segmental artery leaving dorsal aorta 



Figure 6-37. Vertebral elements of an adult lamprey, Pefromyzon monnus, in relation to otfier 

 structures. (After Goodrich, 1930) 



per segment, sometimes extending a .segment and a lialf, and, 

 to confuse things even further, the elements of a pair may be 

 of different sizes. Dorsal and ventral elements are separa- 

 ted from each other by a thin perichordal layer of con- 

 nective tissue. The interdorsal arch lies behind the ventral 

 root foramen, while the dorsal root foramen, is at its apex. 

 The neural arch has a slight diapophysis and its ventral 

 point marks the position of the myoseptum. 



The "vertebrae" of Hydrulagus lack bodies. However, the 

 sheath of the unconstricted notochord is invaded by rings 

 of sclerotomic cells which later calcify. Such rings are lack- 

 ing in Callorhynchus. 



The anterior end of the column of Hydrolagus is fused into 

 a single block which supports the dorsal fin and its spine. 



The units in this anterior piece are somewhat shortened. 

 This anterior section articulates with the cranium by way of 

 a heterocoelous surface on which the notochord is constricted 

 to a fine strand. 



The relationships of dorsal elements to nerve roots in the 

 embryo suggest the agnath but do not resemble the situa- 

 tion observed in other groups. It is quite possible that this 

 nerve-arcualia relationship is indeed basic, but there is no 

 convincing evidence to support the assumption. 



The manifold evidence of specialization — the many vari- 

 ations of structure in different sharks and rays, and the 

 anterior section of the column of Hydrolagus — warns against 

 considering the vertebral type of the Chondrichthyes as 

 primitive. The same remark applies to the lamprey. 



interdorsal 



ligament. 



myoseptum 

 bosidorsal 



ventral root 



dorsal root 



spina! cord 



limit of 

 notochord sheath' 



ligament 



neural canal_ 



dorsal aorta 



postcardinal interventral 



sensory ganglion 

 spinal nerve 



calcified area in notochord sheath 



blood vessels 



B 



Figure 6-38. Vertebral structure of the shark Squo/us. A, in lateral view, anterior vertebra 

 cut obliquely; B, median sagittal section. {After Marinelli and Strenger, 1959) 



THE VERTEBRAL COLUMN 



165 



