Turtles, crocodiles and the external structure of birds

A turtle’s body is enclosed above and below in a shield of bone and horn called the shell, joined to the vertebrae, ribs and collarbone. When in danger the turtle pulls its head, legs and tail inside. It has no teeth, its jaws have horny plates, the neck is long and movable, and sight and smell are good. Most live on land; some in fresh water (webbed toes) or in the sea (paddle-like limbs). In our deserts and foothills the Central Asian tortoise eats grass: it is active for only 3–4 months in spring; when plants dry up in early summer it goes into summer sleep and often passes straight into winter sleep without waking. This species is declining and needs protection. Crocodiles look like giant lizards, grow several metres long, have thick armoured skin, swim with a long, sideways-flattened tail and have webbing between the toes. The eyes and nostrils sit on bumps on top of the head, so in water the animal is almost invisible while it watches for approaching prey. The female digs a pit on the shore, lays dozens of eggs, often guards them and leads the young to water. A crocodile is built more complexly than other reptiles: its lung has many chambers and its heart has four chambers (the blood mixes only slightly). Reptiles arose about 300 million years ago from amphibians: as the climate dried the skin thickened, the lungs enlarged, and the egg shell thickened so eggs could be laid on land. Today the largest crocodile is known to be the saltwater crocodile, over 6 m long.