The 'sea squirt' (or Ascidian, belonging to the subphylum of the Tunicata) is an aquatic animal that in its first stage of life develops into a tadpole-like creature. It swims around until it finds a suitable rock or coral reef to settle in for life. Then the sea squirt digests its own tail and the part of its central nervous system that allows it to move around. In other words, it devours its own brain.

(Thanks to Alex Voorhoeve!)