[dodobirdlord]

An animal’s gut is also arguably “external”, in the sense that the gastrointestinal tract means an animal is a topological torus, and the contents of the gut are also “outside” of the body. It’s an open question how the closed digestive tract evolved, but it presumably came from a digestive surface of some sort folding over on itself, since it must have evolved in creatures that were previously able to digest. The same general phenomenon presumably produced lungs by cavitating a gas-exchange surface to increase surface area.

[thaumasiotes]

Well, a digestive surface is necessarily external in that sense, since the only things you want to digest originate outside the body. The options are that you have a digestive surface that is open to the world, or that you have one which is protected within your interior, but nevertheless topologically external.

Or, I guess, that you have a topologically external surface which does nothing but create vacuoles around food, which then get transmitted to a topologically internal digestive surface. (Similar mechanisms do exist for other purposes; one thing that may happen to a splinter embedded in your finger is that the body applies a protective coating to the splinter and then expels it.)

Starfish seem like an interesting case here, in that they have an ordinary internal stomach which they extrude, when they're eating, for the purpose of external digestion.

[dotancohen]

If you have further reading material on these two developments, I'd be thrilled to go over them. The subject is fascinating, but layman subject matter is scarce and I already have a profession.