380 ORGANS OF DIGESTION. 



gums and other parts to which they adhere, till maturity, 

 when they often anchylos'e to the bones beneath by the 

 ossification of their pulp. They often adhere to the tongue 

 of fishes as in many gasteropods and birds, to the vomer as 

 in amphibia, to the palatine bones as in serpents, to the 

 pharyngeal bones and the branchial arches, as well as to the 

 maxillary and intermaxillary bones to which they become con- 

 fined in the saurian reptiles and mammalia. The dermal teeth 

 spread over the surface of the body in the acanthocephalous 

 entozoa become in like manner gradually restricted to the 

 mouth and jaws in higher forms of helminthoid and entomoid 

 articulata. Some species of fishes, and of higher vertebrated 

 classes, are entirely destitute of teeth ; they are most com- 

 monly placed in numerous contiguous rows, and are fre- 

 quently and variously renewed during life ; they are sometimes 

 confined t6 the jaws and lodged in alveoli, when the new 

 teeth are developed behind and displace vertically the old, as 

 in the crocodilian reptiles. These prehensile osseous hollow 

 spines or fangless teeth of fishes are not opposed to each 

 other so as to serve for mastication, but are most frequently 

 placed alternately, and recurved as in serpents, crocodiles, 

 dolphins, and most other predaceous non-masticating ver- 

 tebrata, to check the escape of their prey or to tear it to 

 pieces. The simple teeth are perforated for the blood- 

 vessels and nerve, and expand over the pulp at their base. 

 In many of the rays they unite to form continuous patiline 

 plates covered with enamel, and in some osseous fishes 

 the anchylosed bases of successive vertical teeth accumulate 

 to form elevated cones continuous with the jaws. The teeth 

 of the tetrodon succeed each other from behind, and the 

 same is observed in the component layers of those of diodon 

 which grow from interposed pulpy laminae at their base. 

 Where the food consists of very soft or minutely divided 

 substances the teeth are sometimes wanting, as in the 

 sturgeon, and they have broad strong crowns in those which 

 break hard substances as the testaceous coverings of mollusca 

 or Crustacea. 



The tongue, almost destitute of gustatory papillae, is 

 broad, short, and cartilaginous or muscular in fishes as 

 in cephalopods, amphibia and many reptiles, and is often 

 covered with teeth supported by the large hyoid bone, and 



