. Chordate morphology. Morphology (Animals); Chordata. 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
. Chordate morphology. Morphology (Animals); Chordata. 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. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Jollie, Malcolm. New York, Reinhold
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