. The chordates. Chordata. 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 approxim
. The chordates. Chordata. 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—, 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,
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