The primary difference between vertebrates and invertebrates is the presence of a backbone or spinal column. Vertebrates, encompassing groups like mammals, birds, and fish, possess an internal skeletal structure, which provides support and protection for vital organs. In contrast, invertebrates, such as insects, arachnids, and mollusks, lack a backbone and may have external skeletons or soft bodies. This difference leads to significant variations in their anatomy, physiology, and ecological roles, influencing their evolutionary pathways and biodiversity in ecosystems.