. Chordate morphology. Morphology (Animals); Chordata. 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 m
. Chordate morphology. Morphology (Animals); Chordata. 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—
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