Why are they vertebrates?

Please provide me with the context! I need to know who "they" are in order to explain why they are vertebrates.

For example, are you asking about:

* Specific animals? (e.g., "Why are cats vertebrates?")

* A group of animals? (e.g., "Why are mammals vertebrates?")

* A characteristic? (e.g., "Why are animals with backbones vertebrates?")

Once you tell me who "they" are, I can give you a detailed explanation!