The Inner (Alimentary) Tube and Its Respiratory Derivatives 



55 



of these embryonic rudiments of teeth has been interpreted as indi- 

 cating the polyphyodont ancestry of mammals. 



Exceptional Dentitions. At one extreme of the vertebrate dental 

 equipment are fishes. In most of them the teeth are small, very numer- 

 ous, widely distributed, of simple form, homodont, and polyphyodont. 

 At the other extreme are mammals, whose teeth are relatively large 

 and usually few, always restricted to the jaws, more or less complex 

 in form, heterodont, and diphyodont or (at least approximately) 

 monophyodont. The general trend is from one extreme to the other. 

 But all along the line there are exceptions. The Australian lungfish 

 (Neoceratodus) has only six teeth. Some fishes are toothless — e.g., 

 sturgeons, sea horses, pipefish. The adult swordfish is not only tooth- 

 less but scaleless. Some fishes are heterodont. In Neoceratodus the upper 

 front pair of teeth are small and bladelike. A pair behind these and a 

 similar pair in the lower jaw are large, flattish, ridged crushers. The 

 wolf fish, cunner, and tautog have front "canines" and back crushers. 

 The scup and sheepshead have large, chisel-like front teeth and back 

 crushers. Thus equipped, a fish may scrape mollusks or barnacles off 

 a rock with the front teeth and crush them with the back teeth. 

 Among amphibians, frogs have teeth on the upper jaw but none on 

 the lower. The common toad is toothless. Of reptiles, turtles are tooth- 

 less and venomous snakes are heterodont, having certain pairs of front 

 teeth differentiated as poison-fangs (Fig. 54). The fang is long, 

 conical, and sharp-pointed. It is traversed by either a superficial longi- 

 tudinal groove or an enclosed canal. The venom, secreted by a pair of 

 glands on the upper jaw, is conveyed by a duct to the base of each 

 fang and passes along the groove or canal into the wound made by the 

 fang. The poison-canal might seem to be a modified pulp-cavity, but 



Fig. 54. (Left) The skull of the rattlesnake. (Gz) Poison-fang; (Rz) reserve fang. 

 (Right) The poison apparatus of the rattlesnake. (A) Eye; (Gc) poison-duct; (N) 

 nostril; (S) poison-sac, which is emptied by constriction of the constrictor muscle 

 (Mc); (z) tongue; (f) opening of poison-duct into canal of fang. (Courtesy, Wieder- 

 sheim: "Grundriss der vergleichenden Anatomie der Wirbeltiere," Jena, Gustav 

 Fischer.) 



