What are invertebrates?

Invertebrates are defined as animals that lack a backbone or vertebral column. This characteristic is the primary distinguishing feature that sets invertebrates apart from vertebrates, which include animals like mammals, birds, reptiles, amphibians, and fish. Invertebrates comprise a diverse group of organisms, including insects, arachnids, mollusks, and cnidarians, among others.

The correct choice highlights this fundamental biological classification, emphasizing the absence of a backbone as a defining trait. This distinction is crucial for understanding the vast diversity of the animal kingdom, as invertebrates account for approximately 95% of all animal species.

In contrast, the other options suggest characteristics that do not accurately define invertebrates. For instance, animals with a backbone refer to vertebrates, while the reference to animals that live in water does not universally apply, as invertebrates can be found in various habitats, including terrestrial and aerial environments. Lastly, stating that invertebrates include birds and mammals is misleading, since both of these groups are actually classified as vertebrates.

Understanding this classification is essential for anyone studying biology or ecology, as it forms the foundation for recognizing the diversity of life and the evolutionary relationships among different animal groups.