Which term refers to animals that lack a backbone?

Which term refers to animals that lack a backbone?

• A. Vertebrates
• B. Parasites
• C. Plankton
• D. Invertebrates

Answer: (D)

Having a backbone is the defining feature that separates vertebrates from other animals. Vertebrates include familiar groups like fish, frogs, snakes, birds, and mammals, all of which have a spinal column. The term for animals that do not have a vertebral column is invertebrates, a huge and diverse group that includes insects, spiders, mollusks, worms, and starfish. Parasites describe a lifestyle—living on or in another organism—rather than skeletal structure, so that word doesn’t indicate backbone presence. Plankton refers to drifting organisms in water and can include both plants and animals; it doesn’t specify whether an organism has a backbone. So the term that best fits “animals that lack a backbone” is invertebrates.