Sea Squirt
Sea squirts are filter feeders that look kind of like inflatable straws. Weirdly, a newborn sea squirt gets to have a brain, but loses its intellect when it grows up. A baby squirt looks like a tadpole and has a teeny brain and one eye, but no way to eat. It swims to the ocean floor where it attaches itself and then grows into an adult, absorbing its now-useless eye, tail and brain in the process.
