Biologists divide living things into groups called kingdoms. This book looks at animals: the life-forms that make up the animal kingdom. Animals vary in body structure, feeding habits, reproduction, and behavior. But all are multi-cellular organisms. All have to eat to get energy, and most have a nervous system that allows them to respond to challenges in their environment. This series covers the fundamental life-science concepts of biology, ecology, evolution, genetics, cell biology, and life-forms, including humans. From the building blocks of cells and the basics of genetic theory through the structure, anatomy, and behavior of living organisms, this series tells the story of life in all its forms.