Animals can be classified using various characteristics. Whether an animal has skin, hair, or feathers depends on the kinds of cells it has. The building blocks of all living things are cells. Scientists are able to study the cells of animals and best group them or classify them. There are several factors that are studied in this classification of animals.
Cells that are alike and perform the same job or function come together and make up tissue. Tissues of different kinds come together and make up an organ, like a brain or heart for example. Finally, a group of organs that work together to perform a certain job is called an organ system. For example, the digestive system of a bat is made up of the mouth, stomach, and intestines. All parts work together to digest food, but each is a different organ.
When cells, tissues, organs, and systems are carefully compared, animals are able to be classified into groups that are closely related. Although they may look very different at first glance, they are classified as closely related by the make-up of the cells, tissues, organs, and systems. A good example would be comparing the wing of a bat to that of an eagle's wing in comparison to the leg of a cat.
At first glance the bat and eagle might be grouped together because they both have wings and fly. However, when closely comparing the make-up of the bone structure, it is discovered that the bat's bone structure closely resembles the leg of a cat where there are five sets of finger bones extending from a longer bone whereas eagles have a single fused bone inside each wing. As a result, scientists would classify bats and cats together because they have a more similar cell, tissue, organ and system make-up than bats and eagles have.
Another group classification depends on whether animals have backbones. Animals that do have a backbone are called vertebrates. The members of the other group that do not have backbones are called invertebrates. And within each of these, whether they are vertebrates or invertebrates, there are even smaller groups divided in order to better classify animals. These classifications range from the simple to the complex.
One of the simplest kinds of animals is a sponge. A sponge's body is like a hollow tube with lots of holes in it with no bone structure. An earthworm is segmented and has eyes, jaws and gills like many other animals yet has no bone structure. Each are classified as invertebrates, and yet there are many further classifications of both sponges and earthworms. The same is true for vertebrates. Fish have a bone structure, yet there are various classifications of fish. Birds have a bone structure and there are many classifications of birds. Both are classified as vertebrates, and yet a fish is much different than a bird and grouped separately.
The scientific study of cells is an endless study needed to classify animals. All animals have cells, tissue, organs, and systems and yet they are classified into different groups. Some are vertebrates while others are invertebrates and still are grouped into smaller categories. The different traits and characteristics of animals are studied and used to classify them into groups. This remains a very complex and necessary scientific study in the classification of animals.
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