"[T]he butthole is one of the finest innovations in the past 540 million years of animal evolution."

"The first animals that arose seem to have literally had potty mouths: Their modern-day descendants, such as sea sponges, sea anemones, and jellyfish, all lack an anus and must eat and excrete through the same hole. Once an independent exit evolved, however, animals diversified into the majority of species alive today, ranging from earthworms
 to humans. One apparent advantage of a second hole is that animals can eat while digesting a meal, whereas creatures with one hole must finish and defecate before eating again. Other possible benefits, say evolutionary biologists, include not polluting an animal’s dining area and allowing an animal to evolve a longer body because it does not have to pump waste back up toward the head...."

From "Why watching comb jellies poop has stunned evolutionary biologists" (via Metafilter).