The Inner (Alimentary) Tube and lis Respiratory Derivatives f>l 



oid scales. The teeth of sharks, in form, structure, material, and 

 manner of development, are essentially placoid scales. 



They differ from scales only in being larger and usually arranged in 

 a series of rows parallel to the edge of the jaw. In some sharks the teeth 

 of successive rows are placed in a pattern like that of scales on the body 

 (Fig. 52). In relation to the grasping and holding of prey, the integu- 

 ment al scales situated over the jaws occupy a strategic position and, 

 accordingly, have become more strongly developed. Depending on the 



-ENAMEL SPINE 



Ftp,. 52. Arrangement of placoid scales 

 in elasmobranchs. The scales are in rows, 

 and usually each scale is in line with the 

 interval between scales of the lines in front 

 and behind. (After klaatsch. Courtesy, 

 Neal and Rand: "Chordate Anatomy." 

 Philadelphia, The Rlakiston Company.) 



dietary habit of the fish, the placoid spines may be so exaggerated as to 

 increase the prehensile value of the tooth-scales, or the spine may be 

 more or less reduced and the basal part of the scale broadened in adap- 

 tation to use for crushing. 



The presence of teeth or scales on the surfaces of the pharynx 

 raises questions. The pharynx is lined by the inner layer of the embryo, 

 the endoderm, which otherwise lines the stomach and intestine. In gen- 

 eral, endoderm does not produce hard calcareous structures such as 

 scales. It is not to be expected that it should secrete enamel. It is, 

 therefore, a question whether pharyngeal teeth, present in many fishes, 



