The main distinction between vertebrates and invertebrates is the presence of a backbone or spinal column. Vertebrates, which include animals like mammals, birds, reptiles, amphibians, and fish, possess an internal skeleton made up of vertebrae that protects the spinal cord. In contrast, invertebrates, such as insects, mollusks, and crustaceans, lack this structure and often have exoskeletons or hydrostatic skeletons. This fundamental difference significantly influences their anatomy, physiology, and adaptations, leading to diverse evolutionary paths and ecosystems.