What animals do not have a brain?

Jellyfish don't have brains

Starfish do not have brains. Instead they have a cluster of nerves called ganglia.

Being more specific than the person before me, I am doing a science course that basically says that there are some animals(E.g. earth worms, shrimp and starfish) that do not have brains per say but they have clusters of ganglia that make a plexus or plexi (the plural form of plexus) and these act as nerve centers.
-Actually, earthworms and shrimp (as do all arthropods) have central brains. However, they also possess a chain of ganglia along their nerve cords. Echinoderms (such as starfish) do not have true brains, instead having nerve nets.