Final answer:
The animal kingdom is primarily divided into invertebrates, which lack a backbone, and vertebrates, with a backbone. These divisions indicate major differences in body structure among the vast diversity of animal species.
Step-by-step explanation:
The animal kingdom may be divided into two large groups of subkingdoms: invertebrates, which lack a backbone, and vertebrates, which have a backbone.
The animal kingdom, or Kingdom Animalia, encompasses a wide array of multicellular organisms that exhibit a heterotrophic mode of nutrition. This extensive kingdom has evolved over millions of years from simple ocean-dwelling creatures to a vibrant diversity of life forms. A key categorization within this kingdom is the division into two large subkingdoms: the invertebrates and the vertebrates. Invertebrates make up the bulk of the animal kingdom, accounting for about 95 percent of all animal species. They are characterized by the absence of a vertebral column, or backbone, and are represented by roughly 32 phyla. Vertebrates, on the other hand, include animals with backbones, such as mammals, birds, reptiles, amphibians, and fish. Although vertebrates are more familiar to us, they represent a much smaller portion of the animal kingdom's species.
These categorizations reflect significant differences in body structure and form, and they play a key role in the scientific classification and analysis of animal diversity. The study of these subkingdoms gives us greater insight into the evolutionary relationships and adaptations that have allowed animals to thrive in countless environments across the globe.