CHAPTER 5 

 TEETH 



No invertebrate has teeth at all comparable, save in function, position, 

 and material, with the teeth of vertebrates. Some annelids, hke Nereis, 

 have horny pharyngeal teeth that act Hke pincers. A circle of calcareous 

 teeth surrounds the mouth of the vegetarian sea-urchin to form the 

 lantern of Aristotle, but each tooth has its own muscles and there is no 

 jaw. Some snails have a radula with which they rasp their food. Many 

 arthropods, notably the biting insects, have their appendages modified 

 into hard mouth-parts that are both jaws and teeth. But a series of 

 independent teeth operated by a movable jaw is pecuHar to chordates. 



Nor do all chordates possess such teeth. The protochordates have no 

 teeth of any sort. Cyclostomes have within the oral hood horny ecto- 

 dermal teeth, with which they cling to their prey or bore their way into 

 its flesh. In this absence of calcareous teeth attached to jaws, as in so 

 many other characteristics, the cyclostomes exhibit their primitive 

 nature. 



The larvae of some amphibia have upon their jaws horny teeth in 

 the form of papillae. Most reptiles have true teeth; but the chelonia 

 have replaced those of their ancestors with horny beaks. So, too, have 

 all modern birds, although their ancestors of the Jurassic and Cretaceous 

 had typical reptihan teeth. The embryo of the duck-billed platypus has 

 rudimentary teeth which it does not use. Even among the placental 

 mammals, the edentates either have no teeth or have them without enamel. 

 In the toothless whales, teeth are present in the embryo, but the adult has 

 only whalebone strainers. 



EVOLUTION OF TEETH 



Since the protochordates are without teeth, the cyclostomes have 

 horny ones, and all attempts to discover any sort of rudimentary cal- 

 careous teeth in cyclostomes have proved unsuccessful, it seems clear that 

 teeth of the vertebrate type are a new acquisition with no homologs 

 anywhere among invertebrates. 



Typical or "true" vertebrate teeth have their beginning in the innu- 

 merable, minute placoid scales, which so roughen the skin of sharks that 

 in former time shark skin, under the name shagreen, was widely used as 

 an abrasive. 



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