Which of the following describes invertebrates?

Invertebrates are characterized specifically by their lack of a backbone or vertebral column, which classifies them as a significant group within the animal kingdom. This group encompasses a diverse range of organisms, including insects, arachnids, mollusks, crustaceans, and more, totaling around 95% of all animal species. The absence of a backbone means that invertebrates generally have different structural support systems, such as exoskeletons in arthropods or hydrostatic skeletons in jellyfish.

Understanding this distinction helps clarify the classification of organisms. The other options refer to characteristics that do not pertain to invertebrates. For instance, the definition of vertebrates describes animals with backbones, while the remaining choices relate to non-animal organisms, such as plants that reproduce by spores or those that cannot produce seeds. Thus, the defining feature of invertebrates as animals without backbones makes the correct answer clear.