294 



SYSTEMATIC ZOOLOGY. 



numerous in the lower and reduced by fusion or actual 

 loss in the higher forms. 



In the sharks the visceral skeleton is very simple, being 

 represented by the upper and lower jaws (Fig. 104, pq, ra) 

 by the gill-arches or giil-bars, and by a few cartilages sup- 

 porting the lips. The upper jaw is not firmly united to 



iv b 



ill 



FIG. 104. Diagram of the skull and branchial arches of a shark, h, hyoid; 

 km, hyomandibular, forming the suspensor of the lower jaw, m (Meckel's car- 

 tilage); pq, upper jaw (pterygoquadrate); s, spiracle; I-V, gill-arches, 

 between which are shown the gill-clefts. 



the cranium, but is held in position by muscles and liga- 

 ments, and by the hyomandibular (hm), while the lower 

 jaw is hinged to the upper, and not to the cranium. Com- 

 parisons, which cannot be described here, show that the 

 upper jaw of the shark is not the same as the upper jaw 

 in the other vertebrates. In them numbers of other bones 

 are added to the skull, and the upper jaw of the shark is 

 only comparable to two pairs of bones, known to anato- 

 mists as the pterygoids and the quadrates (fig. 105). 

 hence the name pterygoquadrate cartilage used for this 

 part in the sharks. 



The rest of the visceral skeleton consists of bars of 

 cartilage on either side of the throat between the gill- 

 slits, the series being united below (Fig. 104). These gill- 

 arches serve to keep this region, weakened by the openings, 

 from collapse. The most anterior of these gill-bars has 

 the special name of hyoid (Fig. 104, h), and its upper part 



