What defines an invertebrate?

An invertebrate is defined as an animal that lacks a backbone or vertebral column. This group encompasses a vast array of organisms, including insects, crabs, jellyfish, and many others. Invertebrates represent the largest group of animals on Earth, making up over 95% of all animal species. Their structural support is often provided by an exoskeleton, hydrostatic skeleton, or other forms instead of a bony backbone. This distinction is crucial in biological classification and helps in understanding the diversity of life forms.

The other options refer to categories or characteristics that do not align with the proper definition of invertebrates. For example, describing a mammal with a backbone does not pertain to invertebrates, as it directly implies the presence of a vertebral column. Similarly, an internal skeleton pertains to vertebrates rather than invertebrates, and plants that reproduce through spores are unrelated to animal classification entirely. Thus, the accurate defining characteristic of invertebrates is their lack of a backbone, as specified in the chosen answer.