18 



Basic Structure of Vertebrates 



Pre. 16. Types of teeth. (A) Pleurodont. (B) Acrodout. (C) Thecodont with 

 open root. (Dj Thecodont with closed root. (Courtesy, Wilder: "History of the 

 Human Body," New York, Henry Holt & Co., Inc.) 



ESOPHAGEAL VILLUS 



Fig. 47. The pharynx of an elasrnobranch (Squalus) laid open to show the 

 double row of teeth in both upper and lower jaws. Such teeth differ only in size 

 from the placoid scales of the pharynx and skin. Elasrnobranch teeth, like scales, 

 are fastened in the skin and are not attached to the jaw cartilages. (After Cook. 

 Courtesy, Neal and Band: "Chordate Anatomy," Philadelphia, The Blakiston 

 Company.) 



