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On a fictional planet far away from our own, there is a part plant, part animal creature of truly massive proportions embedded in the sand of a desert-like portion of the planet, kinda like a cross between Dune's sandworm and Star War's sarlacc. Its mouth remains closed, sitting just below the surface and covered in sand until it senses a victim above it.

At this point, the creature's mouth opens, and the sand falls into the creature's throat, along with any hapless victims that may have been on the sand (kinda like the 'death from below' attack used by a sandworm on a spice harvester in both the trailer of the latest Dune movie and the movie itself). Tentacles within the throat reach through the sand, grabbing all living things within a certain size range (by the way, humans of all ages and sizes fall within the aforementioned size range) and ensuring that anything stuck on, carried by, or attached to the grabbed victims (such as parasites, clumps of sand, or in the case of humans, clothing or equipment) is removed. Once this is done (it typically takes just under a minute), the creature vomits everything not grabbed by the tentacles up out of its mouth and high into the sky above it, and then shuts its mouth. The falling sand then lands on top of the creature's closed mouth, and the trap resets. As an added bonus, anything of value also lands atop the mouth, serving as bait for more prey.

Once this is done, the prey grabbed by the tentacles are then swallowed, but they don't go to the stomach like one would expect. Instead, they are shunted into a sizeable network of cavern-like organs. There aren't any exits swallowed prey can reach, but the prey are pretty much free to wander around in this biological cave system. Even more bizarre, one can actually survive for quite a long while in there; the air quality, while not the best, is still breathable, and one can get all the nutrients they need to survive. Living in this creature is possible, though doing so could potentially be dangerous and certainly won't be pleasant.

Of course, all this raises a pretty big question; What would be a plausible reason for this creature to keep the prey it captured alive and roaming around freely in a network of cavern-like internal organs instead of just digesting its prey?

Note that I haven't decided HOW the thing keeps its prey alive in there, though I'm leaning towards it producing air via low-light photosynthesis and providing saliva for water and nutrient-dense globules of a nectar like substance for food.

The setting is sci-fi, though some “scientifically unexplainable” aspects are acceptable.

Note also that I've yet to establish the thing's origins. It could have naturally evolved, but it could also have been produced by hyper-advanced aliens that specialized in genetic engineering.

EDIT: This doesn't affect many of the answers already out there TOO much, but it's important to note that this thing is STATIONARY. It doesn't burrow around through the sand like a sandworm, it sits in one place like a sarlacc.

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The worm doesn't actually eat its prey. It has low-light photosynthesis, and a lot of body surface, and surprisingly little metabolic need, so it gets by on sunlight and a bit of detritus for nitrogen and phosphorus, somewhere between a plant and a placid ruminant.

Worm larvae, however, are very, very carnivorous. From egg to wormling, they must grow in size by several orders of magnitude. They do so quickly, competitively, and violently. Nothing but the highly calorie-dense flesh of fresh animals is nutritious enough for the little tykes, who hatch inside their worm mother beclawed and ravenous. The details of worm reproduction are too salacious for this website, but suffice to say that opportunities for romance are far and between, and worm mom wants to make sure she's always ready to give her eggs the absolute best start in life - if nothing else to make sure the wormlings, maddened by hunger, don't start eating her from the inside out.

Her bowel-chambers are, effectively, her larder. After all, nothing prevents spoiling better than keeping the captured prey alive. In fact, she can even afford to fatten them with the sugary nectar she secretes (sugar, for a photosynthesiser, is effectively free). Fat is even more calorie-dense than muscle or carbs, and the larvae go absolutely bonkers for bit of grease. Living larders are rather common among animals (wasps being big fans of the technique), and would explain why the worm not only fails to digest her prey, but actually goes through some active effort to keep them alive. Your adventurers will have all the time they need to settle in this smelly, but ultimately comfortable situation.

Of course, at some point, the eggs will hatch.

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    $\begingroup$ Good answer, though it is one that doesn't explain why it lets those it 'eats' roam around freely in its cavernous interior. $\endgroup$
    – Brinstar77
    Apr 11 at 18:59
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    $\begingroup$ I don't think it even realises that they're moving around, to be honest. Do you know what your lunch is up to after you've eaten it? As long as they can't get out, they're effectively captive. $\endgroup$
    – Ottie
    Apr 11 at 19:21
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    $\begingroup$ > It has low-light photosynthesis, and a lot of body surface, and surprisingly little metabolic need... I find this hard to believe. The caloric needs of a creature is partially based on its size. These are huge creatures producing food for its prisoners and even moving through sand (since they eat and it sounds like they reproduce sexually). Photosynthesis is the hardest way to consume energy (that's why most complex lifeforms consume other living things that make the energy for them). $\endgroup$
    – c1moore
    Apr 12 at 3:51
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    $\begingroup$ @Brinstar77 : So larvae can practice hunting and fighting? $\endgroup$
    – vsz
    Apr 12 at 4:37
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    $\begingroup$ @c1moore I actually tend to agree, hence the "surprisingly", but the low-light photosynthesis was in the OP so I ran with it. And it's not clear to me it needs to move much at all - even sexual reproduction could happen via an intermediate "pollinator". Maybe once in a while a particularly feisty captive is unexpectedly released, unaware that embedded under their skin are thousands and thousands of worm, uh, swimmers. You want the feisty ones, you see, as they're the most likely to get themselves caught again. $\endgroup$
    – Ottie
    Apr 12 at 8:30
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Indirect Nutrition and Food Storage

This creature doesn't actually digest the prey, it digests their by-products. It is, essentially, a massive detritivore (eats dead things and feces).

IRL, leaf cutter ants actually eat a fungus that grows in their nests, not the leaves they collect! A similar strategy can apply here—this creature is not collecting food, but a means to produce food.

In more detail, this massive creature would make some sense when it does not drive nutrition directly from what it captures. Instead, it feasts on their by products, like excrement and other decaying matter. The things running around in its gut are just "food storage", like a crop in ants or mother birds.

Speaking of crops, there are many creatures that store food for later somehow. Like an ant's crop, shrikes leaving food on cacti for later, etc. Your creature has a massive crop, which appears to be cave like structures.

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    $\begingroup$ Good point. The creature might even be sentient to some degree in this case, seeing itself like a shepherd protecting a flock, and might even be utterly bewildered by the fact that the humans they 'eat' seem so desperate to find a way out of its insides. $\endgroup$
    – Brinstar77
    Apr 11 at 17:35
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    $\begingroup$ Damn. I was going to say it eats poo. But you got there before me! $\endgroup$
    – Daron
    Apr 11 at 17:49
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    $\begingroup$ The basis of nutrition doesn't really address the caloric balance, or why simply killing them and leaving them to rot wouldn't be a better choice. After the initial capture, all "waste" is partly recovering the diluted investment in producing nectar. $\endgroup$
    – DWKraus
    Apr 12 at 0:35
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    $\begingroup$ @DWKraus that was my initial thought, too. It's wasting more energy by creating the food its victims consume. Ants and humans don't produce the food consumed by their "livestock" they basically just herd the livestock and the food is created by something else (e.g. aphids eat plants, cows eat grass, etc.). $\endgroup$
    – c1moore
    Apr 12 at 3:39
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    $\begingroup$ @DWKraus the creature eats humans for the same reason humans eat yogurt - to help digest something else - fruits, vegetables and ... ahem ... assortment of ground meat. Humans pick up their food from the mouth of the cavern. $\endgroup$ Apr 12 at 6:48
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Predators like humans are a symbiotic part of its immune system.

The giant worm is actually a chemotroph; so, it has no need to actually eat animals at all... in fact it is so big under the surface that eating animals would simply never provide enough food to sustain it. It actually gets its energy from geo-thermal fissures deep under ground. However, this organism is prone to rodent and insect infestations that eat giant worm from the inside out. The worm has no natural defense against these animals; so, they have formed a symbiotic relationship with various predatory species to keep pest populations in check. So when the sandworm gets an infestation, it goes to the surface and waits until it feels an animal moving around above and swallows it, examines it to see if it might be a predator, and if its a predator, it swallows it if it is not, it rejects it.

Once swallowed, the worm has a series of bladders it can inflate or pinch off to lead the predator thorough its body to get it to where the infestations are and then lets the predator eat the parasites.

Part of what makes the inside of a worm so dangerous is not that it is an inherently inhospitable place, after all, the worm wants to keep you alive... it's that the worm tends to gather quite the collection of predators over time. If swallowed you have to be very careful to keep the worm happy with your presence. If you go around trying to eat the worm or starting a fire or what not, then the worm may perceive you as a parasite and start redirecting all the wolves, lions, snakes, etc. that it's also eaten in your general direction.

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  • $\begingroup$ This is the most believable answer I've seen so far. I find it a little hard to believe the amount of control the create has internally, especially combined with the knowledge of what creatures will eat what other creatures (do you know what bacteria in your gut live where). However, we can control our gut biome by how we eat (e.g. eating healthy will increase the population of bacteria in your gut that prefers healthy food). Maybe a more passive approach would be more realistic? $\endgroup$
    – c1moore
    Apr 12 at 4:00
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    $\begingroup$ @c1moore Ah, the old "everything is like a human, no matter the shape" trope :) . Why couldn't a creature develop a better proprioception + control of internal structures? $\endgroup$ Apr 12 at 10:12
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    $\begingroup$ I never said it was consciously doing all this stuff. When you get sick, you eat soup for no other reason than it makes you feel better. You don't need to know how your body sorts out all the immune boosting vitamins, electrolytes, and pro biotics to get things to the right places inside your body. That is just a thing your body does. $\endgroup$
    – Nosajimiki
    Apr 13 at 2:57
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    $\begingroup$ Only a comment since it's only a variant of this answer: The worm uses the swallowed animals like wound-cleaning maggots, for internal wounds. Healthy worm tissue would be too hard to consume for most animals, but sick/dead tissue (for example due to bacterial infection) softens it enough. Removing the dead tissue lets the wound heal faster and helps the worm to fight off the infection. $\endgroup$
    – ooak
    Apr 13 at 4:31
  • $\begingroup$ @DavidMulder is there any creature that has conscious control of their internals? I mean it's not impossible, but generally your internals do what they need to do and there's no conscious effort to make them work. What happens when they sleep? It's a lot of work to keep your internal organs working and evolutionarily usually beneficial not to have to worry about them. $\endgroup$
    – c1moore
    Apr 15 at 16:16
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It is a vehicle.

The organism is a semisynthetic engineered thing; a vehicle for travelling through space and on various planetary surfaces. It is not eating the organisms that come knocking at the door. It is letting them in. The cavern like areas are where the passengers and crew once resided.

The sentients that designed this living vehicle are long departed. This vehicle organism was left where it is. Perhaps it was malfunctioning. Perhaps the original crew and passengers disembarked and did not come back. Over the years it has become buried. It still lets organisms enter.

They can leave too, but not by accident. They need to know how the thing works. It might still be able to do other things too. It has been a long time since anyone has asked it the right way.

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For the same reason why we leave cheese or wine or ham age into humid and cold cellars: it improves their flavor and make it more sophisticated.

The creature is a fine connoisseur of food, and each has its own taste when it comes to seasoning: some like a few days aged "food", some other prefer a more robust, months aged mouthful.

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Can’t get enough of the Stuff

In the movie The Stuff, a calorie-free delicious food-like substance oozes out of the ground, and those that eat it are gradually consumed from within.

Your organism follows the same principle. There is very little energy available to waste on digestion, so this way the food digests itself. All this area is, in fact, a stomach - just not an active one. It can only eat the nectar that “feeds” the prisoners, and the prisoners consume the nectar and are slowly transformed into nectar from within. They start out free- willed, but once the transformation goes far enough, they voluntarily jump into the stomach for the final digestion as the nectar controls their brains. Or, perhaps, they die and dissolve into a puddle of nectar that is absorbed through the "floor." The nectar may provide some caloric benefit, keeping them alive for a time, but eventually it erodes the mind.

The creature recycles some nectar to “feed” the next prey. The nectar takes the place of digestive bacteria in your creature’s food cycle.

  • The nectar may have a narcotic effect, if desired. It may be addictive, so prey don't WANT to leave. The more you eat, the more you rest or pass out, awakening hungry. Or it may be a stimulant, increasing aggression towards any other living things inside the creature (but again, if addictive, no one wants to kill the creature supplying pleasurable nectar)
  • The nectar, as an organism, may provide some nutritional benefit calorically (enough to keep the prey alive until the transformation is complete). Since it is gradually taking over the prey, there is likely little residue. If the organism's chief goal is to obtain trace nutrients from the prey, then keeping prey alive to kill other prey simplifies the process. Dead prey becomes nectar, getting eaten by the creature or by the prey, and concentrating nutrients. Living prey kill others, and if strong enough to survive for long, they eventually become controlled and bring both calories and nutrients to the stomach.
  • If your creature is intelligent, this may be an ethics issue. Perhaps your creature can't ethically kill and eat anyone who doesn't agree to be eaten. By infecting them with the nectar that gradually creates a compulsion to crawl into the stomach and be digested, the creature gains a sort of consent to be eaten from it's prey. While WE wouldn't consider this consent, who knows about the ethics of a giant passive desert predator?
  • Smart and cagey prey might avoid directly eating the nectar, instead consuming the other prey (and avoiding nectar growths in the killed prey). This way, an intelligent prey might live as a hunter semi-indefinitely within the environment of the creature.
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    $\begingroup$ ... don't exactly like this answer. If this were the case, it would be quicker and easier to dump the prisoners in a pit full of the nectar. Besides, it's kinda hard to live long-term inside the creature when the nectar it's feeding you is consuming you from the inside out. Basically, it doesn't explain why the creature lets its prisoners wander around freely in its insides. $\endgroup$
    – Brinstar77
    Apr 11 at 18:15
  • $\begingroup$ @Brinstar77 I've tried to address some of your concerns, but ultimately the energy requirements of the organism must be met by eating the prey. The longer they live, the more calories the prey consume. So unless the prey can somehow culture new calories (like growing plants), they need to be eventually eaten. $\endgroup$
    – DWKraus
    Apr 11 at 23:34
  • $\begingroup$ "Can entropy ever be reversed?" [..] "there was a sudden springing to life of the teletype attached to that portion of Multivac. Five words were printed: INSUFFICIENT DATA FOR MEANINGFUL ANSWER." - The Last Question, Asimov $\endgroup$
    – ti7
    Apr 14 at 20:32
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The worm is a carnivorous geoautotroph

The "cavern" consists of worm appendages that spread out along the hardpan at the bottom of the sand. The "mouth" is submerged just beneath the surface. Between, the immense mechanical strength of the worm's integument allows a spacious conduit for ordinary air (a mixture of oxygen, carbon dioxide, neon, and argon). The worm gets its energy by exchanging heat - its interior is much cooler than the deep sand, and much hotter than the exposed surface of the planet. Note that the serpentine sands of this planet are strongly insulating, allowing a much steeper gradient of heat to build up than is normally seen on Earth.

The planet is significantly less massive than Earth, so nearly all of its hydrogen and much of its nitrogen have been lost. There are no oceans - water is precious and hoarded by lifeforms. There is little nitrogen in the atmosphere, so nitrogen fixation is hard and the element is in very high demand. The worm consumes organisms strictly to maintain their nitrogen reserve for biosynthesis. Since the worm has extensive nitrogen reserves in its body, it normally can leave ingested organisms unmolested.

The possibility of parasitic disease causes the worm to minimize its exposure to the surface air. Anything it eats that doesn't taste like nitrogen will be rapidly expelled, and it normally exchanges heat through a closed mouth, with no direct passages to the surface.

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Those consumed perform a role similar to the micro-fauna within human-range bodies. At the scale of this worm, normal cellular level operations are insufficient.

  • Transferring larger objects
    • Pollination
      • Internal components of this creature are so far apart that reproduction requires "internal fruits" to be produced move things around, or even allow some creatures to escape
    • Circulation
      • Animal-assisted compression of tissues helps circulation, allowing diffusion to play a larger role in resource transfer
    • Excretion
      • Transportation and isolation of toxic byproducts of the worm
  • Interacting with smaller objects
    • Immune System
      • Defending against e.g. a mold inside which releases toxins harmful to both the creatures inside as well as the worm
    • Digestion
      • The natural processes of animals can help produce certain "vitamins" (uric acid, muscle tissue, bones, hair), by breaking down or building up more basic ones. Human-intelligence creatures may also provide more complex reactions, such as fire
  • Meta-operations
    • Nervous System
      • The creatures inside can collectively form triggering mechanisms, similar to hormones or even neurons.
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The consumed are actually forcefully employed as symbiots. The creatures attacked can live within the sarlac-like, but there are other creatures that naturally invade and colonize the sarlac. The "fruit" found inside the sarlac provide adequate nutrition for the symbiots, along with many of the unwanted colonizers also being edible. Ideally, the sarlac is truly looking for creatures that WANT to colonize it, exchanging the unknown random dangers of the surface world for the fairly monotonous dangers of keeping the sarlac healthy.

In times past, perhaps before this region became a desert, there was a surface dwelling species evolved for this, with one of their coming-of-age rituals being the annual "taking below". That species is long gone, and the sarlac adapted by taking a wider variety of creatures, but it's much more of a hit-or-miss scenario now. It's even possible that the symbiot species/race didn't go extinct, but instead simply moved to better grounds. It's possible that forest-dwelling relatives of the desert sarlac-ish still live, but are less well known due to not being a threat. The forest dwelling species still exclusively "feeds" on their approved symbiot, so haven't come under the general scrutiny that the desert variety has.

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It's trying to digest you, but humans are immune to its poison

The organism produces a gas that is poisonous to creatures native to its planet, but humans aren't native to this planet. The gas is unpleasant smelling but very common on earth, so humans won't die from inhaling it.

The creature swallows you into its labyrinthine stomach fully expecting you to die within hours, but if you can stomach the smell you can easily survive by eating the monster's asphyxiating prey.

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The prey aren't the target of the digestion, they're the thing that help digestion move along.

This is mainly inspired by human babies, that may breastfeed even when essentially full or habitually suck on things, specifically to get their digestive systems moving better. So in this situation, if we apply it to the Sarlac-like, we could make a few specific differences that might help:

1.) Carbon dioxide works as a laxative for the Sarlac-like, and while the Sarlac-like generates some carbon dioxide by breathing normally, it tends to exhale it, and not have nearly enough carbon dioxide leftover to help with the digestive process. Humanoids that are captured around the organs, however, will gladly breathe through the leftover oxygen in the canals and produce carbon dioxide, which then filters its way down to the rest of the digestive system, and as a result, they directly help the digestive system. The Sarlac-like then only needs to breathe in and avoid consuming extra oxygen to keep the humans breathing without difficulties.

2.) The act of humanoids walking around, moving about, or reacting to pimples on the inside around the organs causes the humanoids to perform actions that release digestive enzymes that would also end up going down further into the digestive process. This works for the Sarlac-like because they can't reach pimples around in their internal organ areas, but also because humanoids likely panic at seeing human-sized or larger pimples extruding from caverns. The more survivalist and aggressive the humanoid, the better.

Granted, this might be a lot of work for a digestive system, but there is one potential other advantage that a Sarlac-like would find useful in the future:

When they aren't helping digestion move along, they act as a contracted immune system.

3.) The humanoids, especially the survivalist and aggressive ones, are likely to also work additionally as sort of contract white blood cells. As long as the Sarlac-like doesn't let on that they're actually not in as much danger as the humanoids presume, this means they basically get an immune system boost.

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Conserving Energy

This creature is huge. This presents a problem - all of its body needs nutrients and energy, but it takes a whole lot of energy to get the food from the mouth to the tail. A creature that large cannot sustain a full body circulatory system just feeding on scraps. However, if you leave the food alive, it will spread itself out all on its own - it needs to look for food to stay alive, so it has to keep exploring deeper and deeper into the digestive tract. When it finally does starve, wherever it falls, that area of the body will absorb the nutrients from its decaying corpse. Or some other bit of food will come eat it and bring it even deeper.

This can also be used to the worm's benefit deliberately. It has no circulatory system, so it has to use its prey to transport anything that needs to get from point A to point B. Does your tail need potassium? Easy, make some potassium rich hunk of meat fall off of your front stomach, some unfortunate human eats it, and carries the potassium to the back stomach. If it doesn't get all the way, rinse and repeat. Normally you don't want stuff to eat part of your body, but it's a closed system - the nutrients and energy never leave your stomach.

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