132 ELEMENTS OF COMPARATIVE ZOOLOGY. 



anterior of these gill-bars has the special name of hyoid. 

 There is some evidence tending to show that the lower jaw 

 and the palato-quadrate bar are but modified gill-bars. 

 With the disappearance of gills in the higher vertebrates 

 the branchial arches tend to disappear, and in birds and 



FIG. 59. Diagram (after Wiedersheim) showing the relation of permanent 

 structures (dark) to the gill-bars of the embryo (dotted). 7i, hyoid 

 arch ; I, cartilages of larynx ; I, 11, III, gill-bars. At the front of h and 

 I is shown in black the hyoid bone of the adult, with its two horns; be- 

 hind the ear, at the other end of the hyoid arch, is (black) a piece (styloid 

 process) which joins the skull. 



mammals only parts of the hyoid and first gill-bar remain 

 in the adult, where they are largely employed as supports 

 for the tongue. 



There are never more than two pairs of appendages in 

 the vertebrates. These are the fore and hind limbs. In 

 their skeletons these are much alike, and in each can be 

 recognized arches of bone (girdles) uniting the limb to the 

 trunk, and the skeleton of the limb proper. These girdles 

 are known respectively as the shoulder or pectoral and the 

 pelvic girdle. In the fishes the girdles are simple arches, 

 and the skeleton of the limbs is largely composed of fin-rays 

 to support the flattened swimming organ. 



