Does everybody poop? That an animal poops may seem like a no-brainer \u2013 food goes in, gets digested, and whatever wasn\u2019t digested comes out, right? There\u2019s a mouth for food intake and a, you know, a butt, that lets out food waste. A lot of animals don\u2019t have a set up as \u201ccomplicated\u201d at this; scattered throughout the animal kingdom, there are lineages that don\u2019t have a secondary digestive opening (an anus, or a BUTT). In these animals, food is ingested through the mouth, processed by the gut, and whatever hasn\u2019t been digested gets expelled back through the mouth. The anus may not seem like the end-all (ha!) innovation during animal evolution, but it\u2019s actually a pretty big deal. Some evolutionary biologists consider the emergence of the through-gut (a digestive tract featuring a mouth and an anus) to be one of the drivers of animal body plan diversification \u2013 perhaps allowing for more efficient food processing, or providing a stable sort of scaffold to build on.<\/p>\n\n\n\n
Bilateral symmetry, i.e. having a left-half and a right-half of the body \u2013 was another animal innovation that arose after animals had been around for millions of years already. On the oldest branches of the animal tree of life the four non-bilaterian phyla, which split off from the rest of the animal tree prior to the appearance of animals with bilateral symmetry, process food in vastly different ways. Cnidarians (the phylum that includes sea anemones and jellyfish) have a single opening that serves as both a mouth and anus, called a \u201cblind gut\u201d. Sponges and placozoans (tiny animals that look like amoebas) don\u2019t have guts at all. Comb jellies, or ctenophores, have been kind of a question mark as far as gut classification \u2013 in the mid-1800s some scientists reported seeing ctenophores expelling some kind of waste from two pores located opposite the mouth, though this observation was contentious in the field even back then. More recently, the literature about ctenophore digestion has been pretty mixed, with some biologists concluding that ctenophores have a blind gut similar to cnidarians, while others have been unsure of how to classify the ctenophore digestive tract. Maybe it\u2019s a through-gut, maybe it\u2019s something in between a through-gut and a blind gut, who knows \u2013 ctenophores are weird. (They are really, really weird).<\/p>\n\n\n\n