Explain why vertebrates unite in the chordate type.

Vertebrates are combined into the chordate type for the following reasons:
Bilateral or bilateral symmetry and the presence of three germ layers.
There is a secondary body cavity, the appearance of a secondary mouth is characteristic.
Internal axial skeleton in the form of a dorsal string or chord. In the lower (lancelet) forms, the notochord is preserved throughout life, and in the higher forms there is a notochord only in the embryonic period, and then it is replaced by a cartilaginous (fish) or bone-cartilaginous or bony spine.
The nervous system is located on the dorsal side of the body in the form of a neural tube. The neural tube cavity is a neurocoel. In vertebrates, the neural tube differentiates into the spinal cord and the brain.