524 CHOIWATA. 



loosely connected with the cranium as to be easily separated from 

 it. It consists in these forms usually of eight (rarely eleven) 

 arches (fig. 588); these are, from in front backwards, the rudi- 

 mentary labial cartilages, then the large mandibular arch, the 

 Jiyoid arch, and five (rarely seven) gill or branchial arches. The 

 mandibular arch consists, on either side, of two pieces which bear 

 teeth and oppose each other in biting ; the upper half, attached to 

 the skull in front and behind, is the pterygoquadrate (is not the 

 upper jaw of higher forms). The lower part, which is hinged to 

 the other, is the mandibular or MeckePs cartilage. In the same 

 way the hyoid arch is divided into an upper, or hyomandibular, 

 and a lower hyoid proper on either side, the hyomandibular being 

 fastened to the otic capsule. The hyoids are united below by an 

 unpaired piece, the copula. A copula also exists between the 

 halves of the branchial arches, each of which consists of four parts 

 on either side. Hyoid and gill arches bear gills. Certain features 

 (existence of rudimentary gills and a rudimentary gill cleft, the 

 spiracle) indicate also that the mandibular arch was once gill- 

 bearing and that it lost its original function upon being converted 

 into an organ of mastication. Recently the labial cartilages have 

 been regarded as remnants of a support for tentacles around the 

 mouth like those of Amphioxus and Myxine, and which reappear 

 anew in the barbels of bony fishes. Hence they are not compar- 

 able to the other arches. 



By ossification the visceral arches of the higher fishes and all 

 higher vertebrates produce a great modification of the skull, this 

 being increased by a progressive change of function of the arches, 

 which depart more and more from their relations to the respiratory 

 apparatus. From this standpoint they may be divided into two- 

 groups, an anterior, consisting of labial cartilages, maudibular arch,, 

 and the hyomandibular; and a posterior, of the hyoid and the gill 

 arches. The hinder arches are well developed as long as branchial 

 respiration persists. With the loss of gills they largely disappear, 

 but what remains forms the hyoid or tongue bone (not to be con- 

 fused with the hyoid proper), its body being composed of the 

 copula, its anterior horns of the hyoid, and its posterior horns of 

 the remnants of a gill arch. Other gill arches contribute to 

 laryngeal cartilages, the epiglottis and the cartilages of the audi- 

 tory meatus. 



The anterior members of the visceral skeleton (labials, pterygo- 

 quadrate, Meckelian, and hyomandibular) become developed 

 further, but lose more and more their individuality and unite with 



