Main->Readings->5th Grade Readings->Animals Without Backbones->Part 1
Vocabulary |
When you think of an animal, you usually think of something like a cat, a dog, a mouse, or a tiger. Most people do not normally think of a clam, a jellyfish, or an earthworm as an animal. Yet all of them belong to the kingdom of animals.
To a zoologist (a scientist who studies animals), an animal is a many-celled organism, and gets its energy from eating other living or once-living things. Most animals have nerves and can move around at some point in their lives. Scientists think that there are over two million different kinds of animals, and much larger numbers are possible.
Cats, dogs, mice, and tigers belong to a very small group of animals known as mammals. Human beings are mammals as well. Mammals belong to a larger group of animals known as vertebrates, or animals with backbones, which include fish, amphibians, reptiles, and birds as well. Yet the vertebrate animals are only 2% of all the species of animals living on the Earth, with perhaps 40,000 species of vertebrates known. The invertebrates (animals without backbones) make up the other 98%, with over two million species. Insects, the most common type of invertebrate, are the most numerous. Therefore, when you think of an animal, you should probably think of an ant or a bee first, rather than a cat or a dog.
The science of classifying organisms is called taxonomy. In order to study living things, scientists classify each organism according to its:
Kingdom
-----Phylum
----------Class
---------------Order
---------------------Family
-------------------------- Genus
-------------------------------Species
To give an example, here is how human beings are classified:
Kingdom: Animals
Phylum: Chordates
Class: Mammals
Order: Primates
Family: Hominids
Genus: Homo
Species: sapiens
Usually, a species is called by its genus name (capitalized) followed by its
species name (lower case), so a human being is called Homo sapiens. In Latin
that means "wise man."
Scientists divide kingdoms into groups of phyla (the plural of phylum), and phyla are grouped into classes, and so on down the list.
It is not an easy task. In order to group organisms, scientists must look at how they act, how they behave, how they are made, what they eat, how they grow, and what their DNA is like. They also try to guess how the organisms evolved, using fossils to help them.
As you learned in the previous chapter, scientists continue to discuss exactly how organisms should be grouped, and they continue to have some disagreements. In the animal kingdom, they estimate that there are from 30 to 35 different phyla of animals. Only one of these phyla includes vertebrate animals. The rest are all invertebrates.
Each of the phyla has a Latin name. This is partly because educated people spoke Latin when science first began. Today, Latin is also used because science is a world-wide activity which shares its information. Not all people speak the same language. Therefore, scientists agree to use Latin words so that they can understand one another.
The largest and most commonly studied phyla of animals are:
Where did all these animals come from? Scientists believe they probably evolved from the early protists. The cells of animals, like the cells of protists, have a nucleus.
The first fossils of animals appeared over 700 million years ago, starting with animals that may have been very much like the sponge. In this unit, you will read about the invertebrates.
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Go on to Lesson 2: Sponges, Stinging Cell Animals, and Flatworms
This page last modified August 15, 2002