326 



THE RISE OF ANIMAL LIFE 



Fig. 13-45. The turtle is one of the strange animals of 

 the >vorld, yet it is so commonplace that it goes un- 

 noticed. It is completely protected by the shell which 

 has the disadvantage of limiting the movements of 

 the appendages. 



The above picture is that of the box turtle {Terra- 

 pene ornata) and the lower picture is that of the 

 huge green turtle {Chelonia mydas). While the former 

 gets around well on land, the latter is particularly 

 adapted for life in the water. Note how its anterior 

 appendages are adapted for swimming. 



because they are so commonplace we think 

 Httle about them. Turtles have existed a 

 long time on the earth, since the earliest 

 ones were contemporaries of the most 

 primitive dinosaurs. The great reptiles 

 came, and passed on again to extinction, 

 but the turtles have persisted. Even with 

 the advent of mammals the conservative 

 turtle, concealed in its protective armor, 

 has maintained itself and, who knows, may 

 survive long after the mammals, including 

 man, have passed out of existence. - 



The turtle shell is a combination of struc- 

 tures identifiable in other reptiles. It is 



composed of horny scutes similar to the 

 ordinary reptilian scales, with bony plates 

 lying underneath which are fused firmly 

 dorsally to the internal skeleton, including 

 the clavicles, ribs and vertebral processes. 

 A similar plate, the plastron, completes the 

 shell on the ventral side. The combined 

 "box" affords a first-rate exoskeleton into 

 which the animal can withdraw almost 

 completely to shield itself from the out- 

 side world. Such a rigid outcovering has 

 limited its movements to a large extent. By 

 a paddle-like motion of the four append- 

 ao;es it slides along on its ventral side. It 

 moves very slowly and awkwardly so that 

 when it is in danger it merely stops and 

 "pulls into its shell" and outwaits its 

 would-be predator. Body muscles have 

 pretty much degenerated but the leg mus- 

 cles are well developed. Breathing is ac- 

 complished by a pumping action of the 

 neck and leg muscles. 



Turtles live both on land (tortoises) and 

 in the water. Like many other groups of 

 land animals they have returned to the 

 water and have so modified their bodies 

 that they are well adapted to an aquatic 

 existence. The great sea turtles, such as'*' 

 the hawksbill and the green (Fig. 13-45), 

 have their appendages modified into flip- 

 pers. They are never seen on land except 

 during the egg-laying season. Others, such 

 as the common snapping and painted tur- 

 tles, are usually found in water but are 

 also frequently seen on land near bodies of 

 water. Still others, like the high-shelled 

 tortoises, live in certain parts of the world 

 where they have no enemies and grow to 

 enormous sizes, often weighing 600 pounds. 

 These desert forms feed on vegetation 

 alone and rarely if ever take any water. 

 Members of this group have penetrated all 

 habitats from the sea to the desert and have 

 prospered through millions of years. 



The snakes and lizards 



Perhaps the most despised of all animals 

 alive today are the snakes and lizards, not 

 because they are particularly harmful to 



