Which group of animals is classified as invertebrates?

Invertebrates are defined as animals that do not possess a backbone. This classification includes a wide range of animal species, including insects, spiders, mollusks, and other groups. These animals are characterized by their diverse structures and forms since they do not have the rigid skeletal structure that vertebrates have.

Understanding this classification is important in the study of biology because it reflects the vast diversity of life forms and their evolutionary adaptations. For example, invertebrates can be found in numerous habitats, and numerous invertebrate species outnumber vertebrates significantly, highlighting their ecological importance.

In contrast, the other choices listed refer to groups that specifically include animals with backbones, such as fish, amphibians, reptiles, birds, and mammals, which are categorized as vertebrates. This distinction is crucial for understanding the broad classification of the animal kingdom.