286 THE STUDY OF ANIMAL LIFE CHAP. 



It is said that a tortoise walked at least 200 yards, twenty- 

 four hours after it was decapitated, while it is well known 

 that the heart of a tortoise will beat for two or three 

 days after it has been isolated from the animal. In con- 

 nection with their sluggishness it is significant that the 

 ribs which help to some extent in the respiratory move- 

 ments of higher animals are soldered into the dorsal shield, 

 thus sluggish respiration may be in part the cause, as it 

 is in part the result, of constitutional passivity. All the 

 Chelonians lay eggs in nests scooped in the earth or sand. 



The marine turtles (e.g. Sphargis, Chelone), the estuarine 

 soft-shelled turtles (e.g. Aspidonectes), the freshwater 

 turtles (e.g. Emys), and the snapping turtle (Chelydra) are 

 more active than the land tortoises, such as the European 

 Testudo gfceca, often kept as a pet. The tortoise of the 

 Galapagos Islands (Testudo elephantopus), the river 

 tortoise (Podocnemys expansa) of the Amazon, the bearded 

 South American turtle (Chelys matamata), and the green 

 turtle (Chelone my das) attain a large size, sometimes 

 measuring about 3 feet in length. 



Crocodilians (Crocodilia). Crocodiles, alligators, and 

 gavials seem in our present perspective very much alike 

 -strong, large, heavily armoured reptiles, at home in 

 tropical rivers, but clumsy and stiff-necked on land, 

 feeding on fishes and small mammals, growing slowly 

 and without that definite limit which punctuates the life- 

 history of most animals, attaining, moreover, a great 

 age, freed after youth is past from the attacks of almost 

 every foe but man. The teeth are firmly implanted in 

 sockets ; the limbs and tail are suited for swimming, 

 and also for crawling ; the heart is more highly developed 

 than in other reptiles, having four instead of three cham- 

 bers. The animals lie in wait for victims, and usually 

 drown them, being themselves able to breathe while the 

 mouth is full of water, if only the nostrils be kept above 

 the surface. 



8. Birds. What mammals are to the earth, and fishes 

 to the sea, birds are to the air. They are at the climax 

 of activity, as may be inferred from their high tempera- 

 ture, from 2-14 Fahrenheit higher than that of mammals. 



