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With over 2 million different kinds of animals, Kingdom Animalia is the largest of the kingdoms. But when you think of an "animal", what image comes to mind? While animals like bears, fish, and birds are the most familiar to us, over half of all the animals on Earth belong to a group of animals known as "arthropods". Arthropods include animals such as centipedes, crabs, insects, and spiders. This means that the majority of animals come from a group of critters that give most folks the creeps!
So, what exactly is an "animal"? With so many different kinds of animals, it's hard to imagine what they all might have in common. First, animals are "multicellular". This means they are made of many cells, unlike bacteria, which are made of only one cell. Second, all animals must get their food by eating other organisms, such as plants, fungi, and other animals. Plants don't have to eat other organisms because they can use the Sun's energy to make their own food. In addition, all animals need oxygen to survive. Did you know that fish can breathe oxygen from the water that passes through their gills and earthworms get their oxygen through their skin?
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