“One possibility is that the comb jellies evolved through-guts and anuslike pores on their own, independent of all other animals, over hundreds of millions of years. Alternatively, a through-gut and exit hole may have evolved once in an ancient animal ancestor, and subsequently became lost in anemones, jellyfish, and sponges. Perhaps if you’re an anemone or a sponge stuck to a rock, suggests Matsumoto, it’s better to push waste back into the current rather than below.”