Answer the following question given this paragraph: 

Another major step in animal evolution was the evolution of a notochord. A notochord is a rigid rod that runs the length of the body. It supports the body and gives it shape (see Figure below ). It also provides a place for muscles to anchor, and counterbalances them when they contract. Animals with a notochord are called chordates. They also have a hollow nerve cord that runs along the top of the body. Gill slits and a tail are two other chordate features. Many modern chordates have some of these structures only as embryos.


Q: A major step in animal evolution was the evolution of what rigid rod that runs the length of the body?

 Choices:

- limbic system

- endoderm

- mitochondria

- notochord

A:
notochord
