204 



COMPARATIVE ANATOMY 



the notochord of the lower chordates, the hemichorda, urochorda, and 

 cephalochorda. In the cyclostomes, the notochord is still the main 

 part of the axial skeleton. Since the cyclostomes have cartilaginous neural 

 arches, it is probable that neural arches are the earliest vertebral elements. 

 Elasmobranchs, both fossil and modern, show a considerable advance 

 over the cyclostomes. Cartilaginous haemal arches and centra appear, 

 with both neural and haemal spinous processes. The anterior trunk 

 vertebrae of elasmobranchs have short lateral or "costal" processes which 

 extend between the myotomes and which suggest the future ribs of 

 mammals. Since in fossil and living forms two centra may occur in each 

 body segment, and since each centrum usually develops in ontogenesis 

 by the fusion of antero-posterior anlagen, it is possible that two centra 



in each segment (diplospondyly) 

 may have been the original arrange- 

 ment in vertebrates. Elasmo- 

 branchs, moreover, begin the long 

 process of vertebral differentiation, 

 the vertebrae of the tail being 

 unlike those of the trunk, the 

 difference correlated with a differ- 

 FiG. 158. — Sagittal section of Acanthias ence in the relation of the coelom 



vertebrae, cut surfaces obliquely lined. ^^ ^^^ vertebrae. In the trunk 

 c, calcifications of centra; cd, caudmeurals; 



cdh, caudihemals; cr (i), cranineurais region, where the body cavity lies, 



(intercalaria); d exits of dorsal nerve roots; ^^^ haemal arch of each vertebra is 



crh, cranihemals; n, notochord; v, exits 01 ^ 



ventral nerve roots. (From Kingsley's incomplete, while in the caudal 



" Comparative Anatomy of Vertebrates.") ^^^j^^ ^^^^ ^^^^ -^ complete with a 



median spinous process. The notochord persists intervertebrally and the 

 centra are biconcave. The skeleton is still cartilaginous, but the 

 cartilage is often hardened with lime. 



Bony vertebrae make their appearance in Ganoid fishes, some of 

 which however retain a cartilaginous vertebral column. Ball and socket 

 joints between the centra are developed in Lepidosteus (Gar-pike) as 

 in some Amphibia. Amphicoelous or biconcave vertebrae, however, 

 predominate in all groups of fishes. Centra are wanting in the Dipnoi. 



With the amphibia, bone succeeds cartilage; and the vertebrae are 

 differentiated into cervical, trunk, sacral, and caudal. The single sacral 

 vertebra is but slightly modified for attachment to the pelvic girdle. A 

 single atlas represents the cervical series of higher forms. Zygapophyses, 

 for articvdating each vertebra with its two neighbors, first appear in this 

 group. Articulation with the ribs is effected by two sorts of processes,— 

 diapophyses from the neural arches and parapophyses from the centra. 



Lumbar vertebrae are first differentiated in reptiles, which also have 

 two sacral vertebrae. Here also appear vertebrae with centra flattened 



