All parts of the body are built like this. Otherwise we would be blobs.

Before becoming two parts they probably were part of the same parent protobrain tissue.

If you told me the hindbrain was ectoderm-derived but the forebrain was endoderm-derived, I might raise an eyebrow or two.

"Nervous systems are typically defined as networks of neurons that communicate across synapses. But Burkhardt’s lab found something unique in ctenophores: Beneath the animal’s outer surface is a nerve net (pink mesh in this 3D reconstruction) whose neurons are connected by continuous cytoplasm — but with no synapses between them. “I don’t think any other animal has a nervous system like that,” Burkhardt said. Because ctenophores are one of the earliest branches of the animal family tree, the finding indicates that evolution may have crafted nervous systems twice: in ctenophores, and separately in jellyfish and all other animals."