When the average person thinks of a reptile, he thinks of a snake. But actually, this class of animals includes many other creeping and crawling creatures.

In the animal kingdom, reptiles rank between the amphibians and the birds. Amphibians are animals that can live both on land and in the water. As a matter of fact, scientists believe that birds developed from the reptiles several million years ago. At that time, the reptiles were the the ruling class among animals, and they were often of giant size. But these giant reptiles died out, and the reptiles that are living today are comparatively small. The largest of these are crocodiles and the python snakes.

In many ways, reptiles are much like amphibians. All are coldblooded, creeping animals with backbones. They are distinguished mainly by their lungs and their skin. Amphibians breathe through gills when they are young, and later many kinds develop lungs. Reptiles, on the other hand, breathe by means of lungs all their lives.

The skin of amphibians is smooth and clammy, being kept moist by special slime glands. Water passes easily through this skin; therefore, most amphibians dry out and die if kept out of water for long. Reptiles have no slime glands, and their skin is dry and scaly. Because water cannot pass out through their skin, reptiles are able to live entirely on land.

The reptiles living today are divided into four main groups: the turtles, the crocodilians, the lizards, and snakes, and the strange lizardlike tuatara of New Zealand.

Turtles and tortoises differ from other reptiles in having their bodies surrounded by a bony shell covered with horny shields. All turtles and tortoises lay eggs. Many of the turtles live in or near bodies of fresh water, while tortoises live entirely upon land.

The alligators and crocodiles and their relatives are long, fourlimbed animals having scales or plates covering their bodies. Alligators and crocodiles are so much alike that it may take an expert to tell them apart. In the United States, however, the alligators have a shorter and broader snout than crocodiles. Australia has crocodiles, no alligators. but many lizards. Lizards and snakes belong to the highest order of reptiles. The main difference between lizards and snakes is in the structure of the jaws. In snakes, both upper and lower jaws have movable halves with sharp recurved teeth.