Which class of animals is characterized by the absence of backbones?

The correct answer is invertebrates because this class of animals is specifically defined by the lack of a backbone or spinal column. Invertebrates make up a significant portion of the animal kingdom and include a wide range of species such as octopuses, crabs, jellyfish, and many more. Their diversity is vast, and they can be found in various aquatic environments, showcasing unique adaptations for survival in these habitats.

Vertebrates, on the other hand, are animals that possess a backbone, which is a key feature distinguishing them from invertebrates. Aquatic mammals, while they inhabit aquatic environments, are still vertebrates because they have a well-defined spine. Amphibians are also classified as vertebrates; they undergo a life cycle that includes both aquatic and terrestrial stages but possess a skeletal structure that includes a backbone. Therefore, the defining characteristic of invertebrates as a category is the absence of a backbone, making this option the most accurate choice.