Animals with Backbones

            If you could look inside all the animals of the world you would see that some have structures with backbones and some do not. All the different kinds of animals in the world can be classified into two large groups:
1. Animals with backbones (vertebrates)
2. Animals without backbones (invertebrates)
            This is the first big grouping that scientists use. Of course, this is only the beginning of classifying. There are five groups of animals with backbones in their bodies: fish, amphibians, reptiles, birds, and mammals.
            Fish. Fish have backbones inside their bodies. Many other bones in the fish are attached to the backbone. The bones make up the skeleton of the fish. All fish have fins. Most fish are covered from head to tail with hard, overlapping scales. They protect the fish like armor. All fish breathe through gills. They take in water through their mouths and let it out through openings called gill slits. As the water passes through, it flows across the fishes' gills. Molecules of oxygen which are dissolved in the water pass into thin tubes (capillaries) in the gills. Fish are cold-blooded animals. The temperature of fishes' blood is the same as the temperature of the water. Most fish lay uncovered eggs in the water. Young fish hatch out of soft eggs laid in the water. Most fish lay thousands and even millions of eggs at a time.
            Amphibians. There are many different kinds of amphibians (such as frogs, toads, salamanders, and newts), but amphibians are alike in these ways: (1) amphibians are vertebrates; (2) amphibians, when they are young, have gills and get oxygen from the water; (3) amphibians, when they have grown up, have lungs and get oxygen from the air; (4) most amphibians have smooth, moist skins, without scales; (5) amphibians are cold-blooded.
            Fish or amphibian? The Coelacanth has scales, fins, and gills. But the fins are like stubby feet. And the animal has simple lungs as well as gills. Scientists think of this animal as a kind of in-between animal: an animal with some of the structures of amphibians. However, scientists think it is more like a fish and have decided to classify it with fish. Scientists guess that thousands of years ago some animals like these may have become even more like the amphibians we know.
            Reptiles. The fence lizard has a dry, scaly skin. Scientists place this animal in the reptile group. Other reptiles are alligators, crocodiles, turtles, and lizards. Reptiles are cold-blooded. The temperature of a reptile's blood changes with the outside temperature. The reptile is strictly a lung breather. All have bodies that are covered with scales or plates. They lay their eggs on land. The eggs have soft shells. Almost all reptiles live on land all their lives.
            Birds. Birds are the only animals with feathers. Birds breathe air through lungs. A bird's lungs branch into hollow spaces called air sacs in many parts of a bird's body. Air from the lungs is pumped down through the air sacs. In some birds the air sacs reach even into the bones of the toes. All birds are warm-blooded. Birds can live in cold climates and in hot climates. A bird has special body parts and ways of keeping an even temperature. These special ways are adaptations. One adaptation is the way the bird's lungs branch into air sacs. When the bird needs more heat, oxygen for burning can quickly reach its cells through these air sacs. When the temperature is too high, fresh, cool air can flow through the sacs. Water from the body cells can evaporate through the sacs. Feathers are another adaptation that help birds keep a steady body temperature. A bird's feathers trap air in their spaces. This prevents loss of body heat to the outside. The bird's feathers act as insulation. Birds hatch out of eggs with brittle shells. The shell contains food that the young bird inside needs until it is ready to hatch. The shell protects the developing bird from drying out. Most birds can fly. Bird bones are very light. Even a chicken, which is not much of a flier, has some hollow bones. Birds have two wings. Penguins and auks have flipper wings that are good for swimming. Some birds, such as the ostrich, are as tall as eight feet and weigh up to three hundred pounds. Their wing muscles are not strong enough to lift them. Birds have two legs. Different birds have different kinds of feet, depending on the kind of life they lead. Some feet are best for perching, some for grasping, and some for swimming. Birds have no teeth. They have bills that they use for getting food. Some bills are good for cracking seeds, some for digging insects out of bark, some for catching fish.
            Mammals. A mammal is the only kind of animal that has hair. Mammals do not lay eggs. Their young are born alive. Newborn mammals are fed milk that is made in the body of the mother. Mammals are warm-blooded.