The zoology of my setting takes place on a mostly dry planet - it's certainly got rainfall, seas, and wetlands in certain ecoregions, and life exists there mostly, but some things still live in more desert-ified territory. After a major extinction event life has gone backwards a few times and ended up evolving very simple bodied megafauna, ones with decentralized nervous systems, no classic vertebrae, and simple organs - most of them are masses of rubbery tissue with "functional cell-regions." Among these simplicities are a lack of internal digestive systems. Most of them have spongy "feeding surfaces" that secrete digestive enzymes, sopping up what's dissolved off the surface. I've built a LOT of evolution around the use of external digestive systems, it's an integral theme to the food chain and I think I've nailed metabolic diversity in the setting, I just need a reason for it to stick around long enough. I am not looking for reasons as to why this wouldn't work (honestly, the folks on this site need to be a bit more creative.)

I am asking, what selective pressures would avoid favoring internal digestive systems in arid environments that would otherwise cause the enzymes to dry out?

Of course I want to solve the glaring issue as to why this would work and/or how to prevent moisture loss as a result of enzyme secretions. I'm currently working with with ideas around 0% or closer water content in their enzymes, though it might be difficult to make work chemically.

  • 3
    $\begingroup$ Hi Quinn. (a) We allow one and only one question. You're asking two: justify your creature design and explain how the creature won't lose moisture. Those should be separate questions. (b) You appear to be forcing the rules of your planet to accommodate a, frankly, impossible creature. What does it do with the external stomach when it needs to run from a predator? And it would never be truly "exterior" as necessary nerves, blood, and muscles must exist to keep the stomach alive and functioning. I frankly can't even guess why such a marvelous weakness would evolve. Sure you need this answer? $\endgroup$
    – JBH
    Aug 10, 2022 at 11:22
  • $\begingroup$ VTC Needs more focus. We have a strict one question per post requirement. Asking us to brainstorm and generate ideas for you also not suitable for this site. $\endgroup$
    – sphennings
    Aug 10, 2022 at 12:12
  • $\begingroup$ @JBH The digestive surface is part of it's otherwise regular body. Think of it as having a flat face that melts stuff. When predated they are as likely to run off as usual. Additionally I have taken inspiration from real life sources including various echinoderms and cnidarians. $\endgroup$
    – Quinn
    Aug 10, 2022 at 20:52
  • $\begingroup$ It's not two questions, the second is a detail, a consideration, of the first question. "Invent me a weird external digestive surface that preserves water". $\endgroup$
    – Greenaum
    Oct 8 at 6:26

6 Answers 6


Toxic Environment:

Your re-evolved world has evolved desperately toxic plants. Perhaps they utilize alternative amino acids, or heavy metals, or horrifyingly destructive neurotoxins (which might explain the decentralized nervous systems).

Due to evolutionary bottlenecks, none of the existing species are resistant to the complex mess of toxins. But a solution has been found - digest the toxins before ingesting the plant matter! Amino acids are converted, toxins break down, and heavy metals are slowly chelated out by the digestive goo vomited all over the food.

Unfortunately, some parts of the world are too arid and cause the digest to dry out before the food is ready to be eaten. For this, you species evolves an ACTUAL external stomach. The digest forms a hardening, dried coat around the food (a bit like an onion peel) so the moisture is kept in and the enzymes have a chance to work.

If you like, the coating can keep out oxygen. Then, the herbivores can have symbiotic anaerobic bacteria that can tolerate the toxins to break down the toxins for them. Then they spread the bacteria from food to food like a sourdough, letting it ferment until ready to be eaten.

  • $\begingroup$ forming a watertight coating every time it feeds will likely cost more material and water than it gains. A reusable surface would be better. $\endgroup$
    – John
    Aug 11, 2022 at 0:42
  • $\begingroup$ Great answer. It and John's comment inspired me to write another answer. I just wish I could give another +1 for the sourdough part. 😃 $\endgroup$ Aug 11, 2022 at 9:37

Its not always external

External stomach only work with extremely small body size, aquatic habitats, or a fibrous body structure. the bigger an animal is the more surface are it needs for digestion, you cant fit enough externally and not loose a lot more than just water.

you can't have anything describable as an animals with large size and an external stomach that can sustain it without it making up the vast majority of the organisms surface area which means it will lose water no mater what, a digestive organs surface can't be water tight and work. Sometimes the answer to a biology question is you can't without magic.

the only option is to go the starfish route, the stomach is internal most of the time, but the creature inverts it to feed. thus the stomach is only exposed for a very short time. Even then the stomach needs to almost completely surround whatever it is eating. This does mean the creature is immobile when it feeds and it will need to feed for a long time. The stomach needs to completely surround the food source and even then the organism will lose a decent amount of water every time it feeds so it had better be feeding on water rich food sources.

enter image description here

  • $\begingroup$ The star fish is a nice example. $\endgroup$
    – UVphoton
    Aug 11, 2022 at 3:13
  • $\begingroup$ What if its surface predigests and actual metabolism happens internally $\endgroup$
    – Quinn
    Aug 11, 2022 at 7:18
  • $\begingroup$ @Quinn the issue is whatever fluid they excrete is going to evaporate very quickly, its not going to have much time to do anything, All air dwelling organisms that predigest, like spiders, inject the prey using the prey's own waterproof skin as a moisture barrier. $\endgroup$
    – John
    Aug 11, 2022 at 13:11

I think you are stuck with water vapor being lost if it is exposed to the arid environment. But there can be adaptions to reduce the amount of water being lost such as membranes designed for that purpose. If evaporative cooling is not needed for some type skins or membranes you could envision them as moisture barriers.

Also there are people trying to scavenge moisture from the air in desert environments. For inspiration some look to insects and animal as biomimetic examples. So perhaps there are adaptations that your animal has to help collect what little moisture there is in the air.

As a route to evolution I mistakenly thought that sea cucumbers had an external stomach, but it seems that they have a single digestive tube that goes through their bodies and sometimes they expel part of their organs to avoid predators, but that got me thinking that for your creature it has an intake and an output and something in the middle as it takes in its nutrients.

To drive the evolution perhaps it started out as having a mouth that excreted digestive juices to help pre digest what was going through the rest of the system. Perhaps some type of super saliva. It found that that there were nutritional advantages to wrapping its mouth around stuff and letting the digestive enzymes do their work. As time went on the it expanded the area to extract more nutrients and needed more flexibility to wrap around objects and the distance between what was once its mouth and stomach shortened and the stomach and mouth functions kind of merged, and nearby organs like eyes to remain useful moved further away from the mouth/stomach organ. So eventually you end up with an external stomach to have the flexibility to wrap around, access and stay in contact with the food, but there is still a skin or membrane that keeps the juicy bits out of contact with the air.

Or perhaps you could look at amebas or other microorganisms and how/why they surround their food to digest it.

  • $\begingroup$ A digestive surface can't be a moisture barrier and function, but the retracting the digestive organ like echinoderms is a good idea. air dwelling microorganism surround their food, basically they have stomachs. $\endgroup$
    – John
    Aug 11, 2022 at 0:46
  • $\begingroup$ I was thinking digestion side in contact with food, but membrane or moisture barrier on the side exposed to air. $\endgroup$
    – UVphoton
    Aug 11, 2022 at 0:50
  • $\begingroup$ That would work, but it will still lose quite a bit of moisture to leakage unless it can engulf the food entirely. $\endgroup$
    – John
    Aug 11, 2022 at 0:55

What pressures would force most life to possess external stomachs?

Time Pressure

Internal digestive systems are best to avoid moisture loss. But it takes a long time to develop such specialized tissues that you can hold an airtight container inside yourself.

As you say, large complex creatures keep getting wiped out and this leaves large animals that are actually megacolonies of less-specialised cells. Some of the megacolonies are slowly evolving to have more specialised cells, but before they finish they will be wiped out again.

As for how they avoid moisture loss, they form an airtight seal around the food. Much like a SCOBY:

enter image description here

The feeding surfaces only lose moisture when actively feeding. Otherwise they go into sleep mode and dry up. When the colony finds food it sits on top of the food and traps it between the ground and the colony. This stops moisture escaping during the digestive process.

There was a similar thing in the Discovers Channel's Alien Planet where the entire ocean had a big SCOBY on it.

  • $\begingroup$ Sounds good, but for those of us who have (had) no idea what a SCOBY is, giving a quick explanation (even just expand the acronym) and linking to wikipedia would be good to save googling. $\endgroup$ Aug 12, 2022 at 5:02
  • $\begingroup$ It's a "symbiotic culture of bacteria and yeast". Those gross things that people use to make Kombucha, which is a gross drink made by a gross slimy lump of germs that people drink because they mistake a bacterial culture for human culture, and therefore think it makes them cool and interesting to do such a stupid thing. It does not, and I'd show more interest in watching someone just pick up and eat dogshit. Same thing really but a hell of a better story. $\endgroup$
    – Greenaum
    Oct 8 at 6:30

Slurploc Bags

Thanks to DWKraus for the Toxic Environment answer and to John for his comment on that answer about an single use external container being biologically too expensive. Those got me thinking about how best to externally contain the liquids in a dry environment.

Lets take two unpleasant thoughts and combine them.

  1. Flies puke up digestive enzymes on their food and slurp/mop up the externally digested goop with their tongues.

  2. Some spiders immobilize and cocoon their prey so they can snack on them later. In some cases, this involves digestive enzymes being injected into the prey before the slurping begins.

So, icky desert creature gathers plant material or immobilizes prey. Then it spins a watertight cocoon around it, sticks its proboscis in and adds some enzymes to begin the breakdown of the contents into a more nutritious and/or less toxic and/or chocolate flavored and/or less fattening form. This may require several applications of enzymes over time. If necessary, the cocoon can be carried around, hidden, or left in a lair.

When the meal is ready, the liquids get slurped out and the solids are discarded. Then a final small application of a different digestive enzyme allows the cocoon to be consumed (waste not, want not). Alternatively, the empty cocoon might even be reused a number of times before being discarded or consumed.

Edit: How about leftovers?

Size limitations of meal cocoons would primarily be based on how big of a cocoon could be produced (with sufficient strength to contain the contents) and how to carry it.

A cooperative group could make a rather large empty cocoon bag. Plants could be carried back without cocooning. If there's an imobilizing venom (a good idea to keep food fresh longer while not letting it try to chew its way out), the prey could also be carried back bagless. Various plants and animals could even be mixed in the same cocoon to make a stew.

Preserving food while not drying it would be a huge advantage in a desert environment. Even a comatose animal will run out of oxygen or starve eventually, so our creatues need another solution to further extend the shelf life of their food. If evolution gives the right combination, chemicals that could slow the breakdown of early stage external digestion would be added to the arsenal of venoms and digestive enzymes our creatures already possess. This helps considerably with the problem of keeping food fresh longer.

Even better. They've got a preserved stew in the lair and a long day of hunting ahead. Everyone grabs or spins a lunch sized cocoon bag, sucks out enough stew from the big bag, and spits it into their personal bags. Then each one adds the right type and quantity of enzyme to have it be at peak flavor for slurping at lunchtime.

Bon appetit!


How about it's a sort-of colony organism, like a jellyfish? It would produce external "stomachs" that were sort of like it's offspring, or partially so.

The creature comes upon some food. It stretches a fleshy bag around the food, and the bag manages to cover the food, then seal shut around it, using musculature. The bag detaches, then the inside of the bag gets to secreting digestive enzymes, and spends however long absorbing the soup. The nutrients are absorbed into it's flesh.

Then the host creature comes back and picks up the "stomach", it sticks to it's flesh. Sortof like a placenta to a womb. The host and the stomach grow capillaries into each other, and either pass nutrients through the nutrients soaking through flesh between close capillaries, or else they eventually join blood vessels together and go back to sharing the same blood supply. The host gets a handy source of digestion that it can abandon and come back to, the stomach gets the protection and mobility of the host. Symbiosis. They are genetically the same, but different genes are switched on in the stomach, just as they are in your own organs and limbs.

If the host doesn't come back, perhaps it has died, eventually the stomach starts to grow the rest of the parts it needs to become a host / stomach combo itself. So that's also how it reproduces.

The nice thing is that this reflects primitive organisms on Earth, it's the sort of thing that might exist in the sea.

Of course a host will produce several of these stomachs, and have at least a couple available at any time.

Water is preserved by the stomach's tough outer membrane. When the stomach rejoins to the host, perhaps some flap in the membrane opens up to allow them to dock back together, or else the host might dissolve away some of this outer membrane. Perhaps some hormone from the host can cause the membrane to slowly harden and soften as needed.

What pressure causes this? I dunno, why are there jellyfish and sea urchins? As long as there's some advantage to it, it can exist, in the web with all the other living things. Long ago everything was weird colony organisms and primitive animals, maybe vertebrates haven't evolved yet.


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