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)

The main idea here is distinguishing animals by whether they have a backbone. Animals that lack a backbone are called invertebrates. A backbone, or vertebral column, is the defining feature of vertebrates, which include groups like fish, birds, mammals, reptiles, and amphibians. Invertebrates cover a huge diversity—think insects, spiders, mollusks such as snails and octopuses, crustaceans like crabs, and organisms like jellyfish and worms. So when the term asked for refers to animals without that internal skeleton, invertebrates is the right label. The other options don’t fit: a vertebrate is specifically an animal with a backbone; parasites describe a lifestyle that can occur in both backbone-bearing and backbone-less animals; plankton refers to organisms by their tiny size and drifting way of life, not to whether they have a backbone.

The main idea here is distinguishing animals by whether they have a backbone. Animals that lack a backbone are called invertebrates. A backbone, or vertebral column, is the defining feature of vertebrates, which include groups like fish, birds, mammals, reptiles, and amphibians. Invertebrates cover a huge diversity—think insects, spiders, mollusks such as snails and octopuses, crustaceans like crabs, and organisms like jellyfish and worms. So when the term asked for refers to animals without that internal skeleton, invertebrates is the right label. The other options don’t fit: a vertebrate is specifically an animal with a backbone; parasites describe a lifestyle that can occur in both backbone-bearing and backbone-less animals; plankton refers to organisms by their tiny size and drifting way of life, not to whether they have a backbone.