"Principles of Biology: Animal Systems" is a part of the college-level Principles of Biology course series textbooks. It is a tutorial written in questions and answers format to describe the principles of anatomy and physiology in the biological systems, including evolution and the functions of each organ system. This book contains chapters in biological principles, scientific methods, biological system, nervous system, sensory system, motor system, endocrine system, reproductive system, circulatory system, respiratory system, immune system, renal system, digestive system, and origin of species. It is a study guide with in-depth explanations. It is written in question-and-answer format so that it provides students with immediate feedback for every question they may ask on each of the topics. This also provides an interactive dialog with the students so that they are engaged in their learning. It also provides with further explanations of each answer so that students can understand the materials in details. It also provides the students with examples to illustrate the concepts. Each section is written in a modular-unit formtat that is self-contained for easy reading. This allows the students to learn each topic incrementally without relying on re-reading the previous sections. This facilitates the learning style of both sequential-learners and analytical-learners to acquire the knowledge intuitively. The principles and concepts are introduced systematically so students can learn the concept and retain the materials analytically and logically without memorizing the trivial facts. This will help students build the foundation for more advanced level topics to be introduced subsequently.