How is the complexity of the structure of chordates manifested in comparison with invertebrates?

In the process of evolution, chordates appeared. They received an axial cord in their body – a chord. She maintains body position. The chord of the lancelet is clearly distinguishable. It remains with him throughout his life. This is the complicated structure of chordates. But most chordates lose their notochord even at the stage of embryonic development. For example – a person. We have a notochord that transforms into intervertebral discs at the end of the first trimester of intrauterine development. This also applies to many other representatives of the chordate type.