Biology STAAR (State of Texas Assessments of Academic Readiness) Practice Test

The term used for organisms that lack a backbone is "invertebrates." This classification includes a vast array of species, such as insects, arachnids, mollusks, and cnidarians, among others. Invertebrates are characterized by their diverse body structures and adaptations, enabling them to thrive in a variety of environments.

In contrast, vertebrates, which are defined by the presence of a backbone or vertebral column, make up a separate category that includes animals like mammals, birds, reptiles, amphibians, and fish. Chordates refer to animals within a broader classification that includes both vertebrates and invertebrates, as they all share certain developmental features, such as a notochord at some stage of their life cycle. Endoskeletons pertain specifically to a type of skeletal structure found in some vertebrates, providing internal support.

Thus, "invertebrates" is the accurate term for those organisms lacking a backbone, highlighting their unique biological classification and significance in the animal kingdom.