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In my world there is a species (who's name Im still working on) but this species has a built in nursery or terrarium.

This "Terrarium' is found in the stomach where you can actively see the objects. So think if you could see the insides of a human stomach though the base of the skin.

I was wondering how a creature could evolve to have this.

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    $\begingroup$ Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. $\endgroup$
    – Community Bot
    Commented May 21 at 10:50
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    $\begingroup$ Kangaroos, like all marsupials, have a built-in nursery where they nurse their young. $\endgroup$
    – AlexP
    Commented May 21 at 11:06
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    $\begingroup$ There exist many examples of anaerobic non visible terrarium/nurseries for bacteria AKA large intestine. Giant clam, corals or lichens are otherwise closest example that exist where host allows light to reach symbiotes. $\endgroup$ Commented May 21 at 19:52
  • $\begingroup$ I have a grand unifying theory that is based on quantum harmonic resonance via super-entanglement.. which basically describes the whole of reality as a large quantum stomach. The idea I would pass to you (excuse the digestional pun) would be that via entanglement, a ‘stomach’ within one organism could become linked to the operations of another system outside the location of this ‘stomach’. Where the various, ongoing, activities and digestionable solutions and fermentations in one biome have a similar effect on a much larger proxy environment that could be almost anywhere, or where the organism $\endgroup$ Commented May 22 at 15:19

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This will probably never happen

Having anything resembling a terrarium will involve not only a huge amount of energy consumption going into actually making the thing, but also no benefits for the species. As one might expect, it would thus be extremely difficult for this terrarium not to get selected against, because this species will be hunting and gathering themselves to dust trying to get enough food to fuel the production of a terrarium, and they will then be huge reservoirs of energy that other predators can come and attack with ease.

Statistically speaking, it would take a world of virtually-unlimited food and zero predators for this species to be able to do anything exotic whatsoever, terrariums included.

The thing about justifying species evolution

I have read a lot of science fiction, and never have I stopped to question "how and why did that species evolve that way?" Yes, I have stopped to think "how does that species survive in the cold, hard, radiation-filled vacuum of space?", but that's a mechanical question, not an evolution question. If you have a good use for a creature with a built-in terrarium, then put one in and make up a symbiotic/commensalistic organism that causes the production of terrariums in this species while also defending them from threats and feeding them.

Moral of the story: if you try and find out "how could this creature have evolved to have these properties", the answer will universally be "anything exotic or even markedly different from what we have here on Earth will be selected against for one reason or another - either overcomplexity or food requirements - and will die out". Evolution is something that most readers are willing to let you handwave.

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    $\begingroup$ Thank you so much! I'm going to redo this question with some background information about the preset environment I have for more context :] $\endgroup$
    – Caliban
    Commented May 21 at 13:49
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There is no actual example on Earth itself, but here we have the closest thing we can think of: Symbiotic algae. The idea is that a marine algae lives inside the coral polyps. The coral skin allows enough light for photosynthesis to reach the algae and help it work.

In our speculative evolution effort, we can take this one step further. I assume that in planets with a thick atmosphere, buoyancy is achieved by one of the following ways:

1- Scoop the lighter gas molecules and store them in a flotation sac. For instance, on a Venus-like atmosphere, an organism can sequester Nitrogen and use it as a flotation gas, because it is lighter than the ambient carbon dioxide.

2- Chemical reactions generating lighter gas. Water molecules can be broken down and hydrogen fills the flotation bladder. On a Venus-like atmosphere, oxygen can be extracted from carbon dioxide.

The second option is energy-intensive. It requires a source of energy. Animals evolving to use the flotation bladder may either have photosynthesis or simply harbor symbiotic algae. On planets where carbon dioxide is abundant and water is scarce, it is practically impossible to sequester all the carbon to get an Earth-like atmosphere. So, organisms may store oxygen which will serve both as a breating gas and as a flotation gas. The animals would likely be less motile and hence less energetic. They would rely on buoyancy rather than flight. It is possible to speculate that those organisms evolved with time to rely on symbiosis with other more active and aggressive creatures which use this gas bag as their home while at the same time protect it and bring food to their host.

Part of the answer was also inspired by Cristian Cline's book "The teeming universe". The planet Hurcelion is a super-Earth planet, on the border of being classified as a gas giant. A jellyfish-like creature named the "Oone" relies on flotation by filling the bladder with hydrogen. Symbiant creatures, capable of flight are agile hunters which live on its body while at the same time protecting the host. Those creatures normally live on the outside of their host. If a planet has a different atmospheric content (e.g. mostly carbon dioxide and unbreathable), some may evolve to live on the inside and a similar ecosystem would develop in a terrarium-like environment.

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If this nursery evolved from essentially a womb, or a pouch to store their own offspring, and only then became infested with symbiotic organisms, this is not impossible.

The hardest part is the Nursery Sack being translucent: that would have to be beneficial BEFORE it became a terrarium for different species.

The easiest way would be for the terrarium creature to already be near completely translucent for a different reason (there are already fish, squid, octopuses and that are see-through).

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TLDR: you could check out Elysia chlorotica as similar real organism.

I think a wild but maybe not totally impossible justification for a transparent nursery organ could be the need of light to pass through it, as it could be the case for the host of a photosynthetic guest. I think one of the closest real examples could be Elysia chlorotica (https://en.m.wikipedia.org/wiki/Elysia_chlorotica). These are semi transparent sea slugs that eat algae and are able to retain their chloroplasts for a some time in their specialised gut cells. These chloroplast continue to photosynthetize and thus provide the slug with food. This could fit your requirement for a specialized and transparent nursery organ. Although Elysia chlorotica only retains the chloroplasts and not the whole cells or algae, you could change this in your creature. You might also stretch things further and make it a terrestrial creature with a nursery organ containing both algae/mosses and mushrooms/yeasts to provide a broader range of nutrients. The creature would move to different spots and rotate itself to keep it's nursery organ with appropriate light conditions. Even if we stretch reality to make it viable this is a very inefficient way of getting energy and nutrients. The creature will highly likely need other food sources and have a very slow metabolism. Since other food sources would be a lot more effective, the creature would also need to live in a habitat where other food sources are extremely scarce or be outcompeted to extinction.

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The closest I can think of this in the real world is the Surinam toad, who incubates its eggs on its back. For a period of the egg's gestation, the top is, in fact, transparent, and you can see the tadpoles giggling around inside.

Where this differs from a terrarium is that it does not include external plants or air. Doing so would be a little like how a bird's crop works, where they swallow rocks and use them inside a muscular organ like teeth to crush up their food.

So, let's say that the creature ingests seeds that they can't digest, embeds them in the egg when the shell is formed, and includes the chemical seeds for an atmosphere and a medium like dirt. When the eggs are laid, they are embedded in the creatures back, then expand into a bubble of mostly CO2 with the dirt-like medium, seed, and "yolk" at the bottom of the bubble. The plant germinates first, converting CO2 to oxygen so that when the yolk fully germinates, it's wandering around in an enclosed space with its own plant buddy.

I could see this mechanism happening for John Scalzi's kaiju.

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