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.