Susan McMullen, age 11, of Columbia, Tenn., for her question:
HOW MANY KINDS OF REPTILES ARE THERE?
Reptile is a common name given to a group of cold blooded vertebrate animals that include the turtles, lizards, snakes, crocodilians and the tuatara. The family is large. There are about 2,500 species of snakes, about the same number of lizards, nearly 250 kinds of turtles and 25 species of crocodilians. In the whole family there are about 6,000 different species.
You'll find reptiles throughout the temperate and tropical areas of the world. They depend on the environment for warmth since they are cold blooded, and cannot live in areas that are cold.
Almost all of the reptiles are egg laying animals. Oviparous is the technical word. Many of the snakes are viviparous, which means they give birth to living young.
Included with the crocodilians are the alligators, crocodiles, the gavial of Asia and the caymans of tropical America.
The tuatara is a large reptile that looks like a lizard. It lives on the islands off the coast of New Zealand. About 2 1/2 feet in length, the tuatara is the last survivor of a whole group of ancient reptiles.
The dinosaurs were members of the reptile family. They are now all extinct.
Scientists aren't able to say why one branch of the reptile family can die out while another will survive. Snakes and lizards today form the only large family branch. This branch includes about 95 percent of all the different kinds of living reptiles. Fossil remains do not show that they ever held greater importance than now.
The same can't be said of turtles, crocodilians and the tuatara. Fossil beds in many places around the world are covered with shells of many kinds of turtles that are now extinct.
Reptiles are classified midway between the fishes and the mammals. Scientists tell us that the reptiles were the first to break away from the water and live on land.
The Age of Reptiles refers to the prehistoric Mesozoic Era. It was during this time that the reptiles reached their greatest importance. Living then were many kinds of dinosaurs, flying reptiles and other huge reptile forms.
In most snakes and some of the lizards, only one lung is functional. In other reptiles, both lungs are developed equally.
In the snakes and lizards, the thorax and abdomen ware not separated by a diaphragm, and breathing is handled by the muscles of the body wall.
The reptile's heart consists of three chambers, including two auricles and one ventricle. Venous blood returns to the heart from the tail and hind limbs, through the kidneys, by a renal portal system. ::lood from the abdominal region returns by a portal system passing through the liver.
The urinary bladder is present only in turtles and lizards.