basidorsa 



basidorsal 



interdorsal 



/L basi and interventral 



calcified rings in sheath 



basidorsal 



— notochord 



connective-tissue 



calcified ring in sheath 



-notochord cells 

 basi and interventral 



caudal vein 



fused vertebrae 



Figure 6-39. Vertebral structure of the chimaerid, Hydro/agus colliei. A, lateral view of thoracics; 

 B, median sagittal section of thoracics; C, anterior end of column v^ith base of dorsal fin; D, cross 

 section of column in trunk region; E, cross section of column in tail region. 



General observations on vertebrae 



In spite of the conflicting evidence, it seems best to conclude 

 that four pairs of elements (two pairs related to the myo- 

 septa and two pairs of intermyoseptal elements for anticom- 

 pression aid) occurred in primitive forms. From these, the 

 solid-walled neural canals of chondrichthians were devel- 

 oped as well as the open neural canals of teleostome fishes. 

 In teleostomes, the myosepta elements tended to fuse with 

 the interseptal structures, and this was associated with the 

 formation of the vertebral body. 



The vertebral body rose differently in each of the groups 

 examined. It can be assumed that the tendency for mesen- 

 chymal invasion of the sheath was common to choanates 

 (Dipnoi), actinopterygians, and chondrichthians. The verte- 

 bral body of the shark represents one direction of evolution, 

 that of the osteichthian fishes, another. Discrete pleuro- and 

 hypocentral centers are lacking 



In the fishes, the arches rest on the notochord, and they 

 are thus rooted in the perichordal tissue; whereas, in am- 



niotes, these arches chondrify away from the notochord 

 (which is much reduced in size and thus makes room for 

 increased perichordal tissue) and appear to lie outside the 

 body. Although this difference is one which seems scarcely 

 worthy of mention, there has been much concern over 

 whether the vertebral body is "archicentrous" (developed 

 from the arches) or "autocentrous" (developed separate 

 from the arches). 



MEDIAN FINS 



Median fins include the dorsals, the caudal, and the 

 anal — these are structures limited to fishes or at least spe- 

 cialized aquatic forms. The tetrapod groups are not involved 

 in this discussion although it can be noted that whales have 

 dorsal fins and caudal flukes. Similar fins were present in 

 the ichthyosaur reptiles (also mosasaurs and others) with 

 the difference that the caudal flukes of the whale are fleshy 

 transverse planes, while the tail of the aquatic reptiles was 

 like that of a fish. 



166 



THE VERTEBRATE BODY SKELETON 



