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.
According to recent DNA analyses, comb jellies evolved earlier than other animals considered to have one hole, including sea anemones, jellyfish, and possibly sea sponges. But now new video has been taken showing the comb jellies’ indigestible particles exiting through the pores on their rear end.
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.
photo: William E. Browne