What Are Chordates?

Chordates are a very large group of animals with backbones. These animals have a spinal cord inside a backbone or spine, a spinal cord, and an internal skeleton.

A very large group of animals with backbones are called chordates. These animals have a spinal cord inside a backbone or spine, and an internal skeleton. The word chordates come from the word chorda or cord. Chordates are vertebrate animals with a spinal chord inside a backbone, or spine, as part of their nervous system.

A long time ago animals were grouped according to where they lived-land, water, or air. However, today they are grouped according to other similarities, such as how many legs they have, the kind of eyes, their tooth structure, and whether the animals bear live young or lay eggs. This new system of grouping animals considers the similarities between fish, frogs and humans, and places them all in the same large group! So, living things are classified according to the characteristics, which they have in common.

Like all living things, animals are made of cells. Some are simple, one-celled animals, others are made of many millions of cells, and are called complicated animals. Scientists studying living things observe their characteristics to group them. These characteristics include appearance, cells, tissues, organs, and the animals' habitats.

One of the characteristics used by scientists to group many-celled animals is whether the animal has a backbone. If it has a backbone it is called a vertebrate, and belongs to the chordates.

A vertebrate animal has many other bones besides the backbone. There is the skeleton, which, together with the muscles, enables the animal to move around. But vertebrae animals are also divided into two large groups. Those with the changeable body temperatures, and those with steady body temperatures.

An example of an animal with a changeable body temperature would be a fish. Their body temperatures are the same as the temperature of the water in which they live. Fish take water in through the mouth. The gills of the fish take up dissolved oxygen from the water, while at the same time giving off carbon dioxide that dissolves in the water and passes back out through the gill. That is basically how a fish gets oxygen. Female fish lay their eggs in the water, which are then fertilized by a male fish's released sperm cells. Fish can be found in any body of water that has enough food and oxygen. Some fish are in ponds and lakes. Others prefer the running water of streams and rivers, and these fish are called fresh water fish. Still other fish, like tuna and cod are saltwater fish. But there are also fish, like salmon, that can live in either fresh or saltwater. Fish belong to the chordate group of animals.

Amphibians, like fish, have changeable body temperatures, and most of them lay their eggs in the water. Frogs, toads and salamanders are in this class. The way most amphibians differ from fish is, of course, that they leave the water when they become adults.

Young toads and frogs are called tadpoles. In this stage of life, tadpoles get their oxygen just like fish, since they have gills. As they grow and develop, their gills slowly disappear and they get lungs. Salamanders develop the same way, but unlike frogs and toads, they do not have webbed feet. When a lung-breathing animal breathes, oxygen passes through the lining of the lungs into the blood, while carbon dioxide passes from the blood into the lungs to be breathed out. Amphibians have special muscles in their throats that enable them to move air into and out of their lungs.

Reptiles also belong to the group of changeable body temperature animals. Snakes, turtles, lizards, and crocodiles are all reptiles. Reptiles exist in most parts of the earth, except where it gets very cold. All reptiles breathe with lungs during their entire life, and all of them have skin that is covered with scales. A turtle's large scales form a thick covering, which we know as the shell. Both turtles and lizards shed their outer skins as they grow. Snakes shed their entire skins quite often as they grow. In fact, the more food a snake eats, the more often it will shed its skin!



Reptiles lay their eggs on land, covering it with soil or leaves. Most reptile eggs have soft leathery shells, but some have thick skin covering, but there are some snakes born without any egg covering. Reptiles are chordate animals.

Birds and mammals belong to the group of vertebrates, which have steady body temperatures. No matter how warm or cold it is, the temperature of these chordates remains much the same.

How does a bird keep warm? Well, of course, all birds have feathers. Close to the skin are the soft, downy feathers. When a bird fluffs these soft feathers, in cold weather, they trap air, and air is a good insulator. And since water does not penetrate the outer feathers, it helps keep birds warm as well.

To raise their young, most birds build a nest of some kind in which to lay eggs. The male bird often helps to build the nest and care for the young. Each egg contains a cell from which a bird develops. The egg is kept warm by the body of the mother bird. Birds belong to the chordate group of animals.

Mammals also have constant body temperatures. Most mammals grow a coat of hair that is usually called fur. Hair, like the feathers on birds, keeps mammals warm. (Even humans have hairs, which can be seen better through a magnifying glass.) Some mammals, like whales, are kept warm by a thick layer of fat.

Mammals have s special muscle called the diaphragm, which separates the heart and lungs from other parts inside the body. As the diaphragm moves downward, it allows the space in the chest to become larger so air can rush into the lungs. As the diaphragm moves back upward, the space becomes smaller and air is forced out. Of course, this movement is called breathing. All mammals breathe this way.

Most mammals develop from an egg cell that remains in the female's body for some time. Then the young are born without any kind of egg covering. Most mammal babies are born in the spring.

However, the platypus and the spiny anteater are an exception to the rule. They are the only mammals that lay eggs. These egg-laying mammals live only in Australia.

Dogs and cats are mammals. So are cows, bears, pigs, and many other animals. The mothers keep their babies warm and feed them milk. Mammal babies require much parental care, until they can take care of themselves. Mammals are in the family of chordates.

It's truly amazing on how many kinds of animals there are classified as chordates!

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