A major step in animal evolution was the evolution of what rigid rod that runs the length of the body?
mitochondria
limbic system
endoderm
notochord
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.