What term describes an animal that lacks a backbone or notochord?

The term that describes an animal that lacks a backbone or notochord is "invertebrate." Invertebrates make up a vast majority of animal species on Earth, encompassing various organisms ranging from simple forms like sponges to more complex ones like insects and mollusks. Unlike vertebrates, which possess a backbone made of vertebrae, invertebrates do not have this structural feature.

The use of "invertebrate" is essential in biology for classifying animals based on their physical characteristics and evolutionary history. This distinction is significant in understanding the diversity of life and the adaptations that different animal groups have developed over time.

In contrast, the term "vertebrate" applies to animals that do have a backbone, while "mammal" refers to a specific class of vertebrates characterized by mammary glands, among other traits. "Crustacean" is a subclass of invertebrates that includes animals like crabs and shrimp, highlighting that while all crustaceans are invertebrates, not all invertebrates are crustaceans.