. Comparative anatomy. Anatomy, Comparative. 170 COMPARATIVE ANATOMY Among vertebrates, the placoid scales, which first appear in certain sharks of the Upper Devonian, are especially important because of their further evolution. Each of these scales has a flat basal plate of dentine embedded in the skin, and each has commonly also a projecting spine coated with hard enamel like a tooth. From these minute placoid scales of ancient sharks have evolved all the multiform teeth of all the higher vertebrates. From these and other types of scale have evolved also, by simple enlargement, the heavy con
. Comparative anatomy. Anatomy, Comparative. 170 COMPARATIVE ANATOMY Among vertebrates, the placoid scales, which first appear in certain sharks of the Upper Devonian, are especially important because of their further evolution. Each of these scales has a flat basal plate of dentine embedded in the skin, and each has commonly also a projecting spine coated with hard enamel like a tooth. From these minute placoid scales of ancient sharks have evolved all the multiform teeth of all the higher vertebrates. From these and other types of scale have evolved also, by simple enlargement, the heavy continuous dermal armor of ganoids and other. ^^^^ •ENAMEL SPINE. Fig. 128.—The imbricated pattern of placoid scale arrangement in elasmobranchs. The scales are arranged in rows and usually each scale is in line with the interval between scales of the lines in front and behind. (Redrawn after Klaatsch.) fishes. These same bony plates survive also in man and the higher vertebrates as "membrane bones" which, unlike most parts of the skeleton are not pre-formed in cartilage but develop directly in connective tissue, HORNY SCALES Vertebrates, besides bony scales, have also horny; but these have played a much less important part in evolution, and are confined to amniotes, more especially, reptiles. In reptiles, the stratum corneum forms a continuous scaly layer over the entire body, the separate scales being local thickenings which con- tinue to grow by the addition of new keratin from underneath. Serpents commonly shed this scaly coat twice a year. But the rattlesnake retains. 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 Neal, Herbert V. (Herbert Vincent), 1869-1940; Rand, Herbert W. (Herbert Wilbur), 1872-1960. Philadelphia : P. Blakiston's Son
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