Despite lacking a brain, a jellyfish can detect light.

This is due to sets of specialized sense cells that can be found along the outside ridge of a jellyfish’s bell. The kind that are able to sense light act as a primitive set of eyes that help determine which way is up.

Other versions of these cells include a “nose” that senses chemicals in the water and a kind that helps the jellyfish maintain its orientation in the water.

(source)