What term describes an animal that does not have a backbone?

The term that describes an animal that does not have a backbone is "invertebrate." Invertebrates comprise a vast group of animals, including species such as jellyfish, insects, and worms, which collectively make up about 95% of all animal species on Earth. These organisms are characterized by the absence of a vertebral column, allowing for a diverse array of body structures and adaptations that contribute to their survival in various environments. Invertebrates can exhibit a range of forms and can be found across many ecosystems, showing their ecological significance and evolutionary success.

The other choices relate to animals with backbones or specific structural traits, differentiating them significantly from invertebrates.