Chap. 35 REPTILES FIRST LAND VERTEBRATES 725 



side by a bridge of cartilage or bone (Fig. 35.13). The two are usually com- 

 posed of plates of bone overlaid by a mosaic of flat horny scales. In soft- 

 shelled turtles the carapace and plastron are partly bone and covered by a 

 leathery skin (Fig. 35.14). The thoracic vertebrae and ribs are fused to the 

 bony carapace outside the pectoral girdle. It is as if our shoulder blades 

 and collarbones were inside our ribs. Since only the vertebrae of the neck 

 and tail can be moved, the muscles of the body are greatly reduced. Only 

 those of the neck, legs, and tail are well developed. 



The form of the shell varies with the habits of the animal. In land turtles, 

 it is usually high dom.ed and permits the head and appendages to be com- 

 pletely protected as in box turtles; in aquatic species it is low, in the snapping 

 turtle, so small that the head and soft parts are unprotected. The protection 

 afforded by the shell seems to be correlated with the disposition. Most turtles 

 are inoffensive, being structurally set up for retirement to their shells under 

 disagreeable circumstances. On the other hand, those with small or soft shells 

 snap and bite at the slightest excuse. Snappers are ferocious and will strike 

 with the speed and fury of a rattlesnake, without the poison. 



Breathing. The respiratory system is typical of air-breathing vertebrates, 

 with nostrils, pharynx, glottis, larynx, trachea, and lungs — the latter containing 



Radius 



Ischium 



Fig. 35.13. Skeleton of a turtle (Cestudo). The living epidermis outside the 

 bony plates produces the horny shell. During the embryonic development the 

 processes of the vertebrae and the ribs are fused with the bony plates. (Courtesy, 

 Wolcott: Animal Biology, ed. 3. McGraw-Hill Book Co., 1946.) 



