This textbook explores the structure and function of animals. Readers will gain knowledge on the diversity, as well as similarities of animal physiologies -- at the microscopic as well as macroscopic level. Topics include general physiology (tissues and organ systems, sensory reception, respiration, digestion etc.), genetics and reproduction, and evolution. Animal physiology is the study of how animals function. This volume is designed to survey molecular and cellular physiology as well as the major physiological systems and how these systems function to maintain homeostasis in various environments.