. Chordate morphology. Morphology (Animals); Chordata. dentine ^neck canal bony base without enclosed cells Figure 8-29. A and B, sections of scales of Nostolepis from the Upper Silurian (after 0rvig, 1957). C, section of scale of Aconthodes, Penn- sylvanian, (after Goodrich, 1907). the scale; the base of the scale is penetrated by radiating Williamson's and Sharpey's canals. The former were oc- cupied by processes of the osteoblasts of the bony base. The acellular material of the Acanthodes scale contrasts with the cellular nature of the Nostolepis type. In terms of their scales the acanthodi
. Chordate morphology. Morphology (Animals); Chordata. dentine ^neck canal bony base without enclosed cells Figure 8-29. A and B, sections of scales of Nostolepis from the Upper Silurian (after 0rvig, 1957). C, section of scale of Aconthodes, Penn- sylvanian, (after Goodrich, 1907). the scale; the base of the scale is penetrated by radiating Williamson's and Sharpey's canals. The former were oc- cupied by processes of the osteoblasts of the bony base. The acellular material of the Acanthodes scale contrasts with the cellular nature of the Nostolepis type. In terms of their scales the acanthodians are thus of two types. Chondrichthyes The placoid scale of the shark has already been described (Figure 8-22). A point of special interest is the nature of the enamel. This has been called "fibrodentine" since it is formed by calcification of the fibrous material formed be- tween the epidermal enamel organ and the dentine (Figure 8-,30). This material occasionally encloses odontoblasts and is penetrated by the terminal tips of the dentinal tubules. Among sharks there is a great variation in scale structure. By way of contrast, the scale of a ray, Dasybatus, is broad and flat with an acellular bony base (Figure 8-31). The dentinal layer is supplied by branching vascular canals which penetrate the base and the neck of the scale. The bony base shows concentric laminae and is perforated by canals of Williamson. These vascular canals also contain the bone cells of the base. Sharpey's fibers penetrate the base and anchor the bone to the fibrous dermis. Cellular bone is lacking in living chondrichthyes but was present perhaps in early fossils (edestids of Permian). oniceilulor glond stratum germinolivum enomel. germinal layer of epidermis Figure 8-30. Development of placoid scale of the shark. A, early stage in Squa\us (after Klootsch, \ 894); B, later stage in Hepianchus (after O. Hertwig, 1 874). SCALES 233. Please note that these images are extracted from scanned page
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