Which term refers to animals that lack a backbone?

Step 3: Consider the definitions of 'Heterotrophs' and 'Autotrophs.' Heterotrophs are organisms that consume other organisms for energy, and Autotrophs are organisms that produce their own food through processes like photosynthesis. These terms are unrelated to the presence or absence of a backbone.

Step 4: Recognize that 'Invertebrates' is the term specifically used to describe animals without a backbone. This matches the question's requirement.