. The chordates. Chordata. Basic Structure of Vertebrates SP'NE peNT1NE _ ENAMEL. ORGAN. Fig. 51. Section of the skin of an elasmobranch, showing five stages in the development of a placoid scale. (After Scbimkewitsch. Courtesy. Neal and Hand: '"Chordate Anatomy," Philadelphia, The Blakiston Company.) sharks the basal plates are more or less reduced. In a given species of elasmobranch, the teeth on all regions of the jaw are of the same form. Sharks1 teeth are subject to loss and replacement. Successive rows of new teeth are continually developing in the oral membrane posterior (ling
. The chordates. Chordata. Basic Structure of Vertebrates SP'NE peNT1NE _ ENAMEL. ORGAN. Fig. 51. Section of the skin of an elasmobranch, showing five stages in the development of a placoid scale. (After Scbimkewitsch. Courtesy. Neal and Hand: '"Chordate Anatomy," Philadelphia, The Blakiston Company.) sharks the basal plates are more or less reduced. In a given species of elasmobranch, the teeth on all regions of the jaw are of the same form. Sharks1 teeth are subject to loss and replacement. Successive rows of new teeth are continually developing in the oral membrane posterior (lingual) to the rows of functional teeth (Figs. 48, 50). The whole dental equipment of the jaw, including its several rows of functional teeth and, posterior to them, several rows of teeth in various stages of develop- ment, is slowly and constantly moving forward and outward. As old teeth arrive at the outer edge of the jaw, they are shed. Meanwhile, behind them new teeth will have emerged and moved forward into functional position. This replacement apparently goes on continuously throughout the life of the fish. The foregoing description of the structure and development of a shark's tooth would apply, with no important modification, to any one of the scales which are distributed over the external surface of the body (Fig. 51). In most sharks the skin is thickly beset with small scales whose sharp-pointed spines, sloping backward, project above the surface of the epidermis (Fig. 52). In some cases there is no sharp demarcation between the teeth on the jaws and the somewhat smaller placoid scales on the skin just external to the jaws. Very commonly, too, more or less numerous small placoid scales are scattered over the internal surfaces of the mouth and pharynx (Fig. 47). Development of calcareous scales in the skin is a common charac- teristic of fishes. The skin of ancient sharks was beset with "dermal denticles," presumably similar to the placoid scales of modern shark
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