. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. A Jack of All Trades 229 nate because they must work out their salvation by learning how, and have been endowed with the capacity to do it. 2. Scales (a) Fishes. While every class of vertebrates except cyclostomes has some representatives with a scaly skin, the presence of scales may be regarded as the most notable modification of the integument of fishes and reptiles. Whenever fishes lack scales, as for example many Siluridae and certain bottom-feeding fo
. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. A Jack of All Trades 229 nate because they must work out their salvation by learning how, and have been endowed with the capacity to do it. 2. Scales (a) Fishes. While every class of vertebrates except cyclostomes has some representatives with a scaly skin, the presence of scales may be regarded as the most notable modification of the integument of fishes and reptiles. Whenever fishes lack scales, as for example many Siluridae and certain bottom-feeding forms, it is to be regarded as a secondary modification and not the primary ancestral condition. Even in the case of the apparently naked eels, tiny vanishing scales appear for a time in the embryonic stages. There are at least four general kinds of fish scales of particular interest to the comparative anatomist, namely, placoid, ganoid, cycloid, and ctenoid, not including bony dermal plates that reached a high degree of elaboration in extinct ostracoderms (Fig. 17), and other armored fishes of early geologic times. The most primitive fish scales are placoid, appearing first in the ancestral sharks of the Upper Devonian times, and found today among selachians generally. In structure a placoid scale consists of a somewhat flat, basal plate originating from the corium and embedded in it, and usually carrying a spiny projection of toothlike dentine capped with a harder substance con- sidered by some observers to be enamel, formed by epidermal cells, but regarded by others as a special type of dentine, produced by dermal cells. The obvious transition in both structure and position from placoid scales of the skin on the outside of the head, to the rows of teeth within the inner margin of the shark's jaws is so continuous and unmistakable that _ 7- teeth may be regarded as modified placoid scales (Fig. 169). The basal dermal plate of the scale cor- responds to the root of a tooth, whil
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