The Sea Squirt:
This little guy is hermaphrodites (has both male and female reproductive organs) so when it reproduces it releases both sperm and eggs into the water where they develop into a tadpole-like larvae while floating in the water for about three days.
This tadpole has a brain (be it a very small one) which enables it to swim around and see (with its one eye). However in tadpole form the sea squirt is not able to eat so it must quickly find a spot on the bottom of the ocean to settle because sea squirts are filter feeders. Once it has found this spot it will never move again. Once rooted there the sea squirt will begin to absorb all of its organs and parts that it no longer needs and transform them into new parts. It ends up absorbing its eye, tail, and brain. It also converts its gills into intake and exist siphons that will enable it to feed.
So essentially the sea squirt ends up "eating" its own brain once it is properly positioned on the bottom of the ocean and lives out the rest of its life without a brain.