What's fascinating here is that tunicates (like sea squirts) are thought to be the closest relatives to vertebrates, closer even than amphioxus. They just went down a radically different path, perhaps because their chemical defenses against predation are so effective, so they could brutally optimize away things like brains.
"The juvenile sea squirt wanders through the sea searching for a suitable rock or hunk of coral to cling to and make its home for life. For this task, it has a rudimentary nervous system. When it finds its spot and takes root, it doesn't need its brain anymore, so it eats it! It's rather like getting tenure.” - Daniel Dennett