Organ Systems of the Late-Multicellular/Macroscopic Stages

This post is essentially a thread dedicated to the concept proposed in the thread/line of posts linked below, I just want to centralize/document it to get more focused commentary on it. I’ll essentially be using the rules established by Buckly to flesh out how organ systems will work: Macroscopic Editor, Progression, and Principles - #12 by Buckly

It will also be using logic found here, describing how we will deal with simulation the metabolic stats of macroscopic organisms: Metabolic Rate System - #5 by Deus

Both of these concepts will be briefly addressed below, but the above links are important if there is more detail needed.

Metabolic Rate System Background

The basis of our metabolic rate system is this…

  • Catch and eat your prey/food, which will become organic matter to be processed. The amount of organic matter extracted depends on how much organic matter is accessible to you, based on teeth/jaw structure and digestive enzymes.
  • Ingested matter becomes nutrients dependent on your digestive rate, which is affected by your circulation rate.
  • Nutrients turn into ATP/energy dependent on your respiration rate, which is affected by your circulation rate.

We already have several stats to work off of based on this system…

  1. Digestive Speed - How fast ingested material is broken down into nutrients. Oftentimes inversely related to digestive efficiency (faster means less efficiency), though not necessarily.
  2. Digestive Efficiency - How many nutrients you get per ingested material. Oftentimes inversely related to digestive speed (more efficiency means slower digestion), though not always. Important for organisms which eat foods with low bioavailability (less nutrient density), such as plant-like autotrophs.
  3. Circulation Rate - How quickly and efficiently material circulates throughout your entire body, which influences respiration and digestion.
  4. Respiration Rate - How efficiently you extract oxygen/oxidizing gas from the environment.
  5. “Ingestion Percentage” - How much of a food source you are able to ingest. Some material is inaccessible due to inadequate adaptations, such as not strong enough teeth or jaws.
  6. Nutrient Storage - How many nutrients you are able to store.
  7. Ingested Matter Storage - How much ingested matter you are able to store.
  8. Hydration Rate - How quickly water drains.
  9. Hydration Storage - How much water you are able to store.

So, we can already start envisioning some of how we will deal with organs and their effects on some important stats which can effect gameplay. First, we should probably focus on how organs and organ systems as a whole will behave.


This is a rundown of how the organ system will work as a whole, established by Buckly in the post I linked above. Note that my system might autocorrect “metaball” to “meatball”, so if you see the word meatball in here, just know I meant to say metaball (unless otherwise noted).

Each metaball will have a set amount of organ slots. You will start with 3 per metaball. In order to place an organ, you must place it within an organ slot.

There will be several organ systems. The primary organ systems, and the ones I am currently focused, are the nervous, digestive, respiratory, and circulatory organ systems. We can expand to include other systems as we see fit – the endocrine system might be a worthwhile addition – but let’s start with the fundamentals first.

Every organ system has a “root” or “basal” part from which all other organs are derived. These basal organs all take up a single slot, but more complex organs derived from the base part can take up more slots. You can modify every type of organ to a certain extent, even the basal organ.

Chaining consecutive organs belonging to the same system can result in bonuses, but can also come with some detriments. So, connecting two digestive organs together can provide a slight boost to digestive efficiency, but will slightly reduce digestive speed. The detriments for most organs being linked together won’t be too noticeable at first, but might result in important factors to consider as your organism gets more complex.

Certain conditions might be met to confer bonuses as well. For example, having a digestive organ in every slot throughout the entirety of your body will represent the evolution of a complete digestive system, resulting in a considerable boost to digestive efficiency with minimal impact on digestive speed. Bonuses and qualifying conditions will behave differently across different organ systems.

Fundamental Roots

Note that your digestive and respiratory systems will originate from the same root…

Cavity (One Organ Slot)

Indicates the existence of a tube or pouch within your organism. Provides a very small boost to digestive and respiratory stats. Can be upgraded to become a digestive cavity, which indicates the root of a digestive system, or can be utilized as a root to the respiratory system.

Your first cavity placed must be at the anterior end, indicating the start of your organism’s mouth.

Now, let’s get started with discussing how each organ system can work…


Organs forming and derived from the digestive system generally center on bonuses to digestive efficiency, digestive speed, and ingested material capacity. Generally, adding more digestive parts boosts efficiency and storage, but also slows down the speed of digestion.

Conditional Benefits: Having a cavity throughout every metaball alongside the torso of your organism will result in a complete digestive system, providing a considerable bonus to digestive efficiency. This represents the evolution of an anus (don’t laugh), meaning your waste is no longer excreted from the same orifice from which you intake ingested material (don’t laugh).

Basal Part

Digestive Cavity (1 Organ Slot, Replaces Cavity)

a simple tube/pouch. It must be connected to a mouth opening, meaning the digestive system will start near the front of your organism and evolve to go deeper towards the anterior of your organism.

  • Each part slightly increases digestive efficiency and ingested material capacity, and slightly decreases digestive speed (though not to a significant extent).
  • Derived from a basal cavity. Replaces the cavity, meaning you cannot attach a respiratory organ to the cavity now.

Digestive cavities, being the root, will generally have an okay boost to digestive efficiency with a weaker effect on digestive time. We don’t want digestive speed to slow down so quickly since there would be more complex parts for the player to evolve, so these represent low benefit but minimal cost roots.

Derived Parts

Stomach (2 Organ Slots) – Replaces digestive cavity. Considerable boost to digestive efficiency and ingested matter storage, with small but notable increase to digestive time.


  • Increase Storage: Increases the amount of ingested matter which can be stored, but decreases digestive speed.
  • Enzyme Concentration: Allows you to alter the balance of enzymes within your stomach, allowing you to either specialize around a single diet or adopt a more expanded diet, perhaps permitting an omnivorous palate.

Intestines (2 Organ Slots) – Replaces digestive cavity. Strong boost to digestive efficiency (greater than stomach), but no impact on ingested matter storage, and considerable increase to digestive time. Consecutive intestines provide an adjacency bonus (two intestines together provides a bonus to each intestine).

Cecum (1 Organ Slot) – Attaches to any digestive organ. Small boost to ingested matter storage and small increase to digestive time. Primary effect varies, but two options are available…

  • Reduced Dehydration Rate – The cecum works to extract more moisture out of food, reducing the rate at which you lose thirst.
  • Increased Digestive Enzyme Concentration – Works to offer more enzymes to a digestive organ, allowing you to either expand your diet or increase efficiency without adding an entire stomach. Boost not as strong as adding a stomach/intestine.


The respiratory organ system will focus on your respiration rate and other important stats, such as stamina. Similar to the digestive organ system, organisms will have a basal capacity of respiration at the start of the multicellular. However, this basal capacity will be a bit higher than the digestive system’s capacity, allowing the player a bit of time before they have to focus on enhancing their respiratory capacities.

Conditional Benefits and Detriments: Respiratory organs placed close to your mouth will receive a bonus.

Additional respiratory upgrades/parts have reduced marginal benefit, as there is such a thing as “too much oxygen” (this post here has some information: Metabolic Rate System - #11 by rsaavedra). Your 4th gill will not provide as much of a boost as your 1st gill.

Basal Parts

Gills (1 Organ Slot or External Appendage)

Gills provide a decent boost to your rate of respiration. They can be either internal or external.

External Gills: Do not take up an organ slot, potentially freeing up space for another important organ to take up. External gills by default are less efficient than internal gills; they also notably decrease health and environmental tolerance ranges. They can be customized to have an increased surface area which could lead to an external gill being more efficient than an internal gill, but effects to health and environmental tolerance ranges are exasperated. They do not have to be attached to a cavity.


Surface Area: External gills can have their surface area adjusted. Increasing surface area increases the rate of respiration, but decreases health and environmental tolerance ranges. Decreasing the surface area has the opposite effect.

Internal Gills: Take up an organ slot, but provides a stronger boost to respiration rates. They have less of an effect on environmental tolerance ranges, and do not have an effect on health. They must be attached to a cavity.


Pseudotrachae: Reduces respiration rate of the gill, but allows your organism to go on land for a little bit before losing respiratory abilities. Note that how we deal with breathing on land and such will have to be discussed eventually, but for now, I want to keep things as simple as possible.

Trachea (1 Organ Slot, Replaces Cavity)

Trachea provide about the same boost to your rate of respiration as gills do. However, tracheas instead allow breathing on air. Once a trachea is placed, the player’s organism must breath air, even if gills are still present. Having both gills and a trachea allows the player to hold their breath underwater for longer.

Derived Parts

Lungs (2 Organ Slots, Derived From and Replaces Gills)

Tremendous boost to ability to breath on land. (this isn’t fleshed out because this is still a while away, I just wanted to list it to show some ways we can expand on the system)

Swim Bladder (2 Organ Slots, Derived From and Replaces Lungs)

Reduces the energy cost of swimming up or down. Makes it impossible to latch onto the ocean floor.


Your nervous system will enhance the capabilities of your organism by increasing the number of organ slots available per metaball and by allowing the development of more complex organs.

When you first become a macroscopic, multicellular organism, you will only have a basic nerve net if you create a nerve cell type in the early multicellular stage. You will have 3 organ slots available initially, but this can be increased through upgrades. As such, the nervous system acts similarly to the nucleus in the Microbe Stage.

There are two ways I can imagine us dealing with the Nervous System…

  • As a Traditional Organ System – By this, I mean placing parts of the nervous system down just like the digestive system. Through this method, you would have to place down a nervous system part on a metaball for the organ slot capacity to increase.
  • As a Non-Part Based System – By this, I mean: instead of making the player place down/upgrade nervous system “parts”, we just let them upgrade a procedural nervous system that we assume is present in the organism, upgrading organ slot capacity for every metaball at once.

I do think it’s best to deal with the nervous system via option 1. The only reason why I am doubtful is because we would need to deal with that slightly differently than how we deal with other organ systems – your nervous system wouldn’t take up a organ slot, since it would be pointless to upgrade organ slot capacity if you just get a slot taken away from the very part you are placing. That is very doable, but we would need to take special care with the UI.

Important Considerations

Here, we have a way in which we can address organs and organ systems in Thrive. By directly addressing aspects of the stats established by the metabolic rate system, we can address various diverse organ systems in as complex of a way as possible. See if the basic ideas proposed here make sense to you.

One thing I would like to bring up; it’s a very likely that the way we deal with metaballs might not ideally line up with the idea of having every metaball have organ slots. We would likely need a good amount of metaballs in the torso, for example, to allow for detail in letting the player sculpt their organism well. Considering this, how might we best represent these “organ slots” in Thrive?

Other organ systems will have to be simulated: the skeletal and circulatory systems, for example. For now, let’s see if the fundamental ideas make sense.

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I’m not really convinced this is the best design. Especially considering metaballs can be different sizes (this is required to be able to make really good looking convolution surfaces from them, when doing the prototype I didn’t add a size slider to the prototype editor but I setup all the code to allow different sized metaballs to be added).

Considering how brain tissue and muscle type creation I’m thinking of making (and already did in the prototype for brain), I’m much more keen on the idea that cell types that the metaball consists of decide which “organ functions” a metaball performs. Though, this needs some further thought but I think something like picking from the functions that the cell type could perform is required. So I guess in the end this is kind of similar to your idea, but there’d be just one organ per metaball, the metaball size would scale how powerful that organ is, and the organ type selectable for a metaball would depend on the organelles in that metaball’s cell type.

I think that having just one organ per metaball is enough as macroscopic starts with 20 metaballs, that’s quite plenty ot add all of the basic organs.

My idea only applies to the “big” organs that are centralized, like a stomach or a brain. Nervous and circulatory systems, I think should be done with a different system, kind of like how a membrane is a separate selection from the placed organelles. I think I see now based on that difference why you’d pick 3 organs to be placed in a metaball as the circulatory system would take up space in like all metaballs.

Okay so this is like placing a basic organ and then having to upgrade it? I’m not yet decided on how I feel about this. I think we should first implement directly adding the final organs and do playtesting to determine how complex that system already is. If there isn’t enough complexity yet, then I think this kind of more indepth system could be warranted, but if the players are already struggling with making connected organ systems even without this system, then it’s probably too much difficulty to dump on the players.


I’m not sure about there being a fixed number of slots for organs. While it’s good that it encourages players to develop a larger and more complex creature, it does present some awkward issues if you want to make something compact like an urchin or crab.

If we want to go the route of limiting organs per part, my recommendation is a soft limit such as a decrease in efficiency in organs based on how many are compacted into one space (it makes them smaller!). Basically we could just add a multiplier of organ efficiency based on the size of the occupied metaball, and subtract from it the amount of organs present.

I admit it’s not a perfect solution but I feel it’s a good place to start from without imposing hard limits.

An alternative could possibly be completely divorcing the organ system from the outward layout of the organism outside of assigning where “orifices” are. You could simply design an organ system and then determine it’s entry and exit points as you prefer. By measuring the distance between orifices connected to the same system you could determine the length of a digestive tract and such.

Such a method would definitely make the later stage editors more approachable, but it would come at the cost of removing the possibility of determining vital parts of an organism’s body or some such. I guess it would probably be much much easier for autoevo to puzzle out too… It also removes any ability to determine whichever part might be necessarily vital to an organism.

I’m not entirely keen on “fixed” organs as it kind of negates the purpose of cell/tissue specialization, and thus removing a large impact of the prior stages from the editor. However… I understand that generalization neglects specification, it is hard to fathom how exactly we can decide what cells create what organs when they can be a huge mess of multiple functionalities so I understand why you went this route.

This is an aspect I feel we should focus on discussing as it is a very integral part of the editor. We need to determine how the player’s cellular composition impacts their organism, and in my mind, the use of cell specialization to enhance and specialize organs makes sense.

A potential solution would be the ability to assign “roles” to these tissue types that then determines how the game calculates their effectiveness score and display to the player, as well as their impact on other organ systems. For instance, a photoreceptive organ given the role of respiration would have a very poor score because it would hardly be respirating.It probably wouldn’t contribute much to photoreception either as it would not be using it’s photoreceptive abilities in the respiratory role.

Honestly I was of the mind that we would start with 1-3 metaballs forming a very basic worm or blob upon reaching macroscopic, with each of the player’s specialized cells becoming organs with no assigned roles. Beginning the stage in the editor as we likely are, we could walk the player through how to assign roles to their organs (If they are not automatically assigned somehow, we’ll need to figure out a way for sake of autoevo), and determine their placement in the organism if applicable.

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This is more like what I’d want. I feel like it is very clunky to try to fit together the overall shape and internal organ systems. The metaball approach works very well for defining the shape of a creature, but for defining organs it is definitely quite limiting and conflicting with the other goals of the metaball system.

This would be pretty arbitrary as quite many players make at least 5 different cell types in early multicellular. Which should the game keep and which the game would automatically just throw out like “lol, you made this? nah you don’t need that, I’m deleting it”?

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I see what you mean, and I agree. Though we should take care as to not make the debuff be “flat”, in that it just is applied to every type of organ which occupies a metaball. For example, your arms - a musculature, circulatory, nervous, and skeletal system is present in a relatively small area. Though if we assign different “sizes” to different organs - for example, making blood vessels have much less mass than a heart - then I guess that is averted.

That then brings up the question of how we will pace progression through the game so that a player doesn’t just evolve a well-developed digestive system right away, though that can be dealt with via the nervous system.

Yeah, that’s why I don’t want to think of a metaball as representing only one organ type because there are layers and layers of organ systems intricately connected all over your body. The arm example above is relevant, but even your torso has massive arteries, bones, organs, etc. all overlapping. Which is why I also agree with:

Because in my mind, metaballs don’t correspond with cell types, but cell types are instead spread across metaballs. Which I realize is basically the antithesis of your prototypes, but I do think having metaballs correspond to a bunch of a specific tissue wouldn’t allow the possibility of organ overlap as discussed above.

We definitely don’t need to make organs bound to metaballs like was discussed in the OP then, I was just assuming it would be an easy way to organize things. I see now that there could be a better way of dealing with things.

Well, it would be easier than a separate system, but if we pick a non-optimal system now (or when initially implementing it), we might be stuck with it forever (or basically forever). So I think separating metaballs+skeleton/joints/muscles from the internal organs system. In the editor we should probably anyway have separate tabs for the structure and internal structure (I think I may have even added the buttons in the prototypes). So the internal structure (organs) editing should really start from the fact that we’ll have a known shape of the organism defined and the job of the organ system is to allow the player to add and customize some internal organs. How that’s best done I’m not sure yet.

But I think that for having reasonable complexity we should not really think about the player having to design the circulatory system or other really tedious parts. Only really like the lungs and digestion systems would be something the player would conceivably would need to design. Everything else just seems pretty obvious to me that we’d rather have just some kind of “how much organ space is dedicated” to specific systems like the liver or circulation.

As a closing thought, I’ll say that this is probably impossible to design well now. This’ll be much more relevant and easier to design once we have more complete metaball editing and joint adding between metaballs to make creatures move. Basically once we have the structural part of macroscopic editor done it’ll be much more clear how the internal editing features can be done.