Basic Structure of Vertebrates 



SP'NE pe NT1NE _ 



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. 



Sharks 1 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 sharks. 

 In all vertebrates the oral cavity is lined by infolded ectoderm (see p. 

 274). The ectodermal oral epithelium is continuous with the external 

 epidermis and is underlaid by a mesodermal layer which resembles the 

 dermis of the external skin. It is evident that the "skin" of the shark's 

 mouth shares, with the external skin, the capacity for producing plac- 



