1 84 



CHORDATE ANATOMY 



of the gills were set free for other uses. Fishes had already demonstrated 

 that a cartilaginous gill arch could be used for seizing food. Land animals 

 turned the remaining arches to other divergent uses. The tongue became 

 attached to the hyoid arch. The hyomandibular element in amphibians 

 became a sound-conducting apparatus. Some of the arches were used to 

 support the vocal cords and the voice-box. 



Amidst all the many adaptive changes which affect the skeleton of the 

 appendages in land animals, both pectoral and pelvic extremities retain 

 their fundamental similarity to one another. Even the differentiation 



'W. °^ 



PRIMITIVE 

 REPTILES 



MAMMAL-LI KE 

 REPTILE 



Fig. 174. — A series of skeletons of hands and feet of tetrapods showing the con- 

 jectured evolution of the human hand and foot. The human hand is evidently less 

 specialized than is the foot. (Redrawn after Romer's "Man and the Vertebrates," 

 University of Chicago Press.) 



of hand and foot does not obscure this fact. The diversities of form and 

 function of the tetrapod appendage do not concern us in this discussion. 

 It is, however, interesting to note that, notwithstanding the high degree 

 of specialization of the extremities of man, they differ little in fundamental 

 structure from those of amphibians. 



To assist their hind legs, amphibians connected the pelvic girdle with 

 the sacrum; to assist their hearing, they converted the suspensorium of the 

 jaw into an ear bone, the columella. Tripartite girdles made their appear- 

 ance in amphibians. The sternum also is a novelty in this group. In the 

 theromorph reptiles, changes in the articulation of the jaw began to take 

 place, so that when mammals appeared, the old hinge between articular 

 and quadrate had been lost, and a new hinge between dentary and 



