What defines vertebrates?

Vertebrates are defined as animals that possess a backbone or spinal column, which is a critical feature that distinguishes this group within the animal kingdom. The backbone is part of an internal skeleton made of vertebrae, which not only provides structural support but also plays a vital role in protecting the spinal cord, a key component of the nervous system. This defining characteristic allows vertebrates, which include groups such as mammals, birds, reptiles, amphibians, and fish, to exhibit a wide range of complex movements and behaviors.

In contrast to vertebrates, the other options describe different biological classifications. Organisms with distinct external skeletons are typically invertebrates, such as arthropods. Animals that lack true tissues would fall under a lower classification, such as sponges, which do not have organized tissue structures. Lastly, single-celled organisms, like bacteria or protists, are fundamentally different from vertebrates, as they do not possess any multicellular complex structures, including backbones. Understanding these definitions forms a foundational aspect of studying biodiversity and the classification of living organisms in environmental science.