400 CHORD ATA 



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

 ribs to the vertebrae. These must be considered as parts of the skull, 

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

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

 lature (myomcric), so the visceral arches are similarly related to the gill 

 formation (branchiomeric) . Like the cranium the visceral skeleton has a 

 cartilaginous and a bony stage. The visceral skeleton is entirely carti- 

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

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

 usually of seven (rarely nine) arches (fig. 546) ; these are, from in front 

 backwards, the large mandibular arch, the hyoid 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 pterygoquadraie 

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

 the other, is the mandibular or Meckel's cartilage. In the same way the 

 hyoid arch is divided into an upper, or hyomandibular, alid 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. 



In front of the mandibular arch, in the Elasmobranchs, are two or three 

 labial cartilages, but it is doubtful if they are visceral arches. Recently they 

 have been regarded as remnants of a support for tentacles around the mouth 

 like those of Amphioxus and Myxine, which reappear in the barbels of bony 

 fishes. 



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 stand- 

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

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

 the hyoid, the gill arches, and the copula-. 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 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- 



