SKELETON. . 63 



The cartilaginous visceral skeleton arises in the pharyngeal region 

 which is weakened by the presence of the gill clefts. It consists of a 

 series of pairs of bars, the visceral arches (fig. 61, /-FIT), lying in 

 the septa between the clefts, the bars of a pair being connected below the 

 pharynx. Each bar, at first, is a continuous structure, but to allow of 

 changes of size in the pharynx, each becomes divided into separate parts, 

 while the arches become connected in the mid- ventral line by unpaired 

 elements, the copulae. The two anterior arches are specialized and 

 have received special names, the first being the mandibular , the second 

 the hyoid arch, the others, in the region of the functional gills, being 

 called collectively gill or branchial arches. The number of these 

 last varies with the number of gill clefts, there being seven in the primi- 

 tive sharks, a smaller number in the higher groups, in which, with the 

 loss of branchial respiration, their form and functions may be altered. 

 At first all are clearly in the head region, but by the unequal growth of 

 cranium and pharynx the gill arches are carried back. All of the 

 arches are cartilaginous at first. 



The mandibular arch lies in the region of the fifth nerve, behind the 

 mouth and between it and the first visceral cleft or pocket, the spiracle 

 or Eustachian tube. The arch is divided into dorsal and ventral 

 halves (fig. 61, /), known respectively as the pterygoquadrate (pala- 

 toquadrate ptgq), and Meckelian cartilages (m). In the elasmo- 

 branchs and chondrostei the pterygoquadrate forms the upper jaw, 

 lying parallel to and joined to the cranium by ligaments or (chimaeroids) 

 by continuous growth. With the appearance of bone a new upper jaw is 

 formed, as described below, and the pterygoquadrate becomes more or 

 less reduced, and ossifies as two or more bones with greatly modified 

 functions. Meckel's cartilage is the lower jaw of the lower vertebrates, 

 while in the higher it forms the axis around which the membrane bones 

 of the definitive jaw are arranged. 



The hyoid arch lies between the spiracle and the first true gill cleft, 

 in the region of the seventh nerve. It divides into an upper element 

 the hyomandibular cartilage (fig. 61, hm), and a ventral portion, the 

 hyoid proper, which may subdivide into several parts (infra). In the 

 lower elasmobranchs the hyomandibular and the rest of the hyoid arch 

 are closely connected, but in the higher fishes the hyomandibular be- 

 comes more separated from the ventral portion and tends to intervene 

 between the mandibular arch and the cranium, becoming a suspensor 

 of the jaws (fig. 63). Still higher it loses its suspensorial functions, 



