Do you know what these greenish, blob-like shapes are? Would it surprise you to learn that they are animals? They don’t look anything like the animals you are probably familiar with—animals such as dogs and deer, fish and frogs. But the greenish blobs are animals nonetheless. They belong to a phylum called Cnidaria, but you may know them as jellyfish. They are very simple animals and not fish at all. How can an organism as simple as a jellyfish be considered an animal? How are animals defined? What traits must an organism have to be classified in the animal kingdom? In this chapter, you will learn the answers to these questions. You will find out just what it means to be an animal.