460 ClIORDATA 



ning of llie alimentary tract and are related to the cranium, much as are 

 riljs to the \"ertebra\ These must he considered as parts of the skull, 

 although in part they are shoxed backwards and lie under the anterior end 

 of the vertebral column. As the ribs arise in alternation with the muscu- 

 lature [niviuiicrir), so the \isceral arches are similarh' related to the gill 

 formation (l>rauriiioiiicri(') . Like the cranium the \-isceral skeleton has a 

 cartilaginous and a lionv stage. The ^■isceral skeleton is entirely carti- 

 laginous only in bdasmobranchs, and here it is so loosely connected with 

 the cranium as to be easily separated from it. It consists in these forms 

 usually of se\'en (rarely nine) arches (lig. 54(1) ; these are, from in front 

 backwards, the large luaiuJibnlar arcli, the liyoiJ arch, and hve (rarely 

 seyen) gill or hraniitial arclics. The maiidibular arch consists, on either 

 side, of two pieces which ])ear teeth and oppose each other in biting: the 

 upper half, attached to the skull in front and behind, is the ptcrygoijuadrale 

 (is not the upper jaw of higher forms). The lower part, which is hinged to 

 the other, is the niandibular or Meckel's caiiilagc. In the same wa-\' the 

 hyoid arch is diyided into an upper, or lixoiiiaiidibiihir, and a loM'cr 

 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 halyes of the branchial arches, each of 

 which consists of four parts on either side. Plyoid 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 orice 

 gill-bearing and that it lost its original function upon being con\-ertetl into 

 an organ of mastication. 



In front of the mandibular arch, in the Fdasmobranchs, arc two or three 

 labial carlihi'^cs. liut it is doubtful if they are \'iscer;d arches. Recently they 

 have been rc^'ardcd as remnants of a sup^iort fdr tentacles around the mouth 

 like those of Ampliioxns and iMyxinc, which reappear in the barbels of bony 

 fishes. 



By ossification, the visceral arches of the higher fishes and all higher 

 vertebrates produce a great modihcation 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 stand- 

 point they may be divided into two groups, an anterior, consisting of labial 

 cartilages, mandibular arch, and the hyomandibular; ami a posterior, of 

 the hyoid, the gill arches, and the copuhe. The hinder arches are well 

 developed as long as branchial respiration persists. With the loss of gills 

 they largely disappear, but what remains f(n-ms the hvoid or tongue Ixme 

 (not to l>e confused with the hyoid proper), its body being composed of the 

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



