Yes, Mr. President, we are all of us donuts

Back in the '60s, so I heard the tale, President Kennedy was visiting Germany and wanted to show his fellow-feeling for the citizens of Berlin.  "Ich bin ein Berliner!" he declared, and the Berliners applauded enthusiastically.  They were making acommodations for this charismatic American, politely ignoring his grammatical mistake that caused the sentence to mean: I am a donut.  As in: I am a hamburger (to say you are a citizen of Hamburg would be "Ich bin Hamburger"--the 'ein' makes all the difference between person and comestible).

But nevertheless, it's true, you know:  we are all donuts.  Animals that are higher than jellyfish and sea anemones--which is nearly the whole the animal kingdom--including creepy crawlies and worms of all kinds (round, tape, earth, lug...) as well as the scaly-feathery-furry sort--animals are all donuts, with an outer outside but also an inner outside, namely the hole running through the middle that is the gut passage. 

Just as we have a layer skin to protect us from all sorts of insults, so we have specialized layers lining our mouths, throats, stomachs, small and large intestines that protects us as needed from what we ingest, and from our own stomach acid for that matter.  Unlike our skin, though, the intestinal lining decides what to allow in as well as what to keep out.  How that works is a fascinating subject which I will explore further another time.  It is where all the trouble with gluten begins, too.  So your gut is not your inside, not quite, even though we are used to thinking of it that way; it's more of a holding tank and processing site.  

By the way, anemones and their ilk have a gut running through their middles too, but they are not donuts because the hole does not go all the way through.  This has the interesting consequence that their mouth has to double as their anus. 

And this business of the donut hole is fundamental to animal architecture.  It may be because a key feature of animals is that they consume other living things.  In embryonic development of virtually all animals, the first hint of structure of the dividing ball of primordial cells that arise from the fertilized egg is when a pouch or dent appears and deepens to form the gut.  And the first major division of animals is based on which opening forms first in embryonic development, the mouth or the anus.  One group is called Protostomes, or First-mouths; the other however is not called Protoprocts or anything of the sort, but rather the more polite "Deuterostomes," or Second-mouths.  Guess which group we chordates, we mammals, we poor clever apes, are in.  Along with our first cousins the Sea Squirts,

and our third cousins the echinoderms, we comprise the Deuterostomes.

While flies and lobsters and all manner of worms and molluscs and strange colonial creatures called bryozoanl are Protostomes.  But the lot of us, barring the anemones who don't know their barf from their fewmets,  are all basically donuts.