What distinguishes invertebrates from other animals?

Invertebrates are characterized primarily by their lack of backbones, which is the definitive trait that distinguishes them from vertebrates, the group of animals that do possess a backbone. This anatomical difference means that invertebrates encompass a diverse range of animals, including insects, arachnids, mollusks, and many others, which are categorized together precisely because of this common feature.

The absence of a backbone allows invertebrates to have a wide variety of body plans and adaptations, enabling them to thrive in various environments. They possess other structures for support and movement, such as exoskeletons in arthropods or hydrostatic skeletons in jellyfish, but these do not constitute a backbone.

Other options relate to characteristics that do not distinctly differentiate invertebrates from vertebrates. For instance, many invertebrates are not necessarily larger than vertebrates, some invertebrates do have complex nervous systems that can be highly developed, and the presence of a nervous system itself is not unique to invertebrates since vertebrates also possess nervous systems. Thus, the most accurate distinguishing factor is that invertebrates lack backbones.