EXISTING ORGANISMS— ANATOMY 



77 



the principal bony framework of the fin is not unhke that of the 

 appendages of terrestrial vertebrates. The fin is articulated to 

 the girdle by a single basal bone, to which are attached two other 

 bones, and following these is a series of bones of less regular 

 arrangement. The fringe of the fin is, of course, a structure 

 adapted to aquatic life, and hence plays no part in comparison 

 with terrestrial forms, but the rest of it foreshadows both in struc- 

 ture and use the conditions found in higher animals. 



The Appendicular Skeleton of Terrestrial Vertebrates. Above 

 the fishes we have to deal only with terrestrial animals and those 



c ^:::^' 



t 





Fig. 52. — Diagram of girdles and appendages from the posterior side; upper 

 letters, fore limb; lower, hind limb. «, acetabulum; c, carpus; co, coracoid; 

 /, femur; _^, fibula; g, glenoid fossa; /;, humerus; i, ilium; is, ischium; ss supra- 

 scapula; mc, to<, metacarpals and metatarsals; p, pubis; pc, procoracoid; ec, 

 epicoracoid; p/i 1-3, phalanges; r, radius; s, scapula; i, tarsus; Ih, tibia; w, 

 ulna; I-V, digits. (From Kingsley's Comparative Anatomy of Vertebrates, 

 with the permission of P. Blakiston's Son & Co.) 



which have become secondarily aquatic, of which there are exam- 

 ples in all classes except the birds. Among birds aquatic habits 

 are always associated with terrestrial and usually also aerial life. 

 Throughout these four classes the girdles still consist of a ventral 

 part extending between the appendages, and lateral parts extending 

 dorsad, which may be attached to the spinal column, as in the 

 pelvic, or supported by muscles as in the pectoral girdle (Fig. 52). 

 The Pelvic Girdle. The pelvic girdle is regarded as probably 

 more conservative (Fig. 53). It consists in the primitive sala- 

 mander, Necturus, of a broad ventral plate of cartilage, bearing 

 two bones, the ilia, running dorsad. The acetaljulum, a cavity 

 in which the hind limb articulates, is located at the junction of 

 the two parts. At the posterior angles of the cartilage plate 

 are two centers of ossification. In higher Amphibia two anterior 



