Which of the following best describes an invertebrate?

An invertebrate is best described as any animal without a backbone, which encompasses a wide range of species. The second choice notes the existence of an external skeleton or shell, which is a characteristic of many invertebrates, such as crustaceans and mollusks. This option highlights a defining feature of certain invertebrate groups, which may possess hard outer structures for protection and support.

The other options focus on characteristics that are not applicable to invertebrates: warm-bloodedness pertains to specific mammals and birds (the first choice), possessing a backbone refers to vertebrates, which specifically excludes invertebrates (the third choice), and land-dwelling mammals pertain solely to a category of vertebrates, leaving out the vast majority of animal life, which includes many invertebrates (the fourth choice). Therefore, the second option accurately captures an essential aspect of invertebrate biology.