. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. 532 Biology of the Vertebrates unconstricted structure in cyclostomes, dipnoans, cartilaginous ganoids, and a few other fishes in which no central rings develop about it. Among the cyclostomes only the lamprey eels have vertebral elements. Throughout most of the trunk region each segment has two pairs of carti- laginous rods attached to the notochordal sheath and extending dorsally alongside the nerve cord (Fig. 428). These elements are probably the homo-
. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. 532 Biology of the Vertebrates unconstricted structure in cyclostomes, dipnoans, cartilaginous ganoids, and a few other fishes in which no central rings develop about it. Among the cyclostomes only the lamprey eels have vertebral elements. Throughout most of the trunk region each segment has two pairs of carti- laginous rods attached to the notochordal sheath and extending dorsally alongside the nerve cord (Fig. 428). These elements are probably the homo- logues of basidorsals and interdorsals. As the rods do not meet dorsal to the nerve cord, the neural canal is roofed over by connective tissue. In the posterior trunk region these structures become reduced and irregular. The absence of these parts in hagfishes is believed to be due to degeneration. Among fishes the vertebrae posterior to the anal region develop haemal arches on the ventral side of the centra (Fig. 432b). The spinal column, therefore, may be divided into trunk vertebrae, essentially alike, which are anterior to the anus, and postanal caudal vertebrae, possessing the haemal arch and diminishing progressively in size toward the posterior end. A slightly modified condition exists in the amphibians, with a smaller total number of vertebrae involved. A single cervical vertebra, providing for the beginnings of independent head movements, is inserted next the skull, while between the trunk and caudal vertebrae there is differentiated a single sacral vertebra (lacking in the limb- Sacral Vertebra - Urostyle^. less apodans) to which the hind legs are attached. The anchorage of the hind legs to this solitary inadequate sacral vertebra is one of the reasons why the amphibians are unable to "stand up" and bear their weight upon their hind legs. Amphibians are the first vertebrates with well developed zygapophyses. In adult frogs the caudal vertebrae are fused toge
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Keywords: ., bookcentury1900, booksubjectanatomycomparative, booksubjectverte
