Anatomically Correct Modular Body Plan Animals

Question

The fixedness of body plans varies widely across different types of Earthling organisms. At one extreme, you have things like tardigrades, for which every individual of any given species has exactly the same number and arrangement of individual cells, differing only in size; at the other extreme, you have things like mycelial fungi, which don't really have any consistent large-scale shape.

At various points in the middle of the spectrum, you have typical animals and plants: all animals of a given species tend to have the same high-level shape (e.g., number and arrangement of limbs), even though they differ in small-scale details and may be different sizes, while plants tend to be similar in terms of the shapes of specific organs (leaves, branching structure, etc.), but can modularly assemble those mid-range features into wildly different large-scale structure--i.e., it is easy to find pairs of animals that are nearly identical to each other, but good luck finding two trees that grew in exactly the same shape! This modular construction conveys numerous advantages to plants, most notably the fact that they can sustain massive injuries and still survive--lop a third of the limbs off a typical tree, and it won't care, 'cause it's got or can grow spares. Lop off a third of a dog, and, well... you've got an animal cruelty case and a lot of blood to clean up.

So, can we make a plant-style modular-type body plan work for more animal-like creatures? How would something like that evolve?

Requirements

A modular animal must:

1. Be mobile.
2. Be heterotrophic--whether vegetarian or carnivorous doesn't really matter.
3. Be constructed largely out of repeatable, interchangeable, and redundant organ complexes, such that damage and regrowth is both possible and expected.
4. Have its detailed large-scale shape determined in large part by environment and injury history, not fixed by genetics.

And for the sake of narrowing the scope:

5. Live on land.
6. Exist in a size range typical of mammals--say, somewhere between a housecat and an elephant.

This does not, however, necessarily mean that a modular animal can't have critical specialized organ systems, like a single head or single digestive tract; after all, separate the crown of a tree from its roots, and it will die (unless each separate part is in good conditions to regenerate the missing half, of course)--each part may be modular, but that doesn't mean the different parts are arbitrarily divisible.

A list of all Anatomically Correct questions can be found here: Anatomically Correct series.

Answer 1

Oops, someone did that (sorta)

I’d like to introduce the siphonophore, a colony organism made of specialised individuals called polyps (or sometimes zooids). Examples include the long strings of polyps.

The hideous mass of many different polyps.

Or perhaps the more familiar, blue bottle jellyfish.

But it isn’t a jellyfish, it’s just related. The sail is an individual organism, and so are each cluster of stinging tentacles, the feeding polyps, and reproductive polyps. Each individual serves one purpose and is utterly useless on its own, having to rely on the other polyps to perform the actions it lacks.

This concept can easily be blown up into more complex (and less gelatinous) colonies. In fact, a certain future biology documentary has explored this a little (https://speculativeevolution.fandom.com/wiki/Ocean_phantom).

The problem of living on land means you may need ‘lung polyps’ ‘locomotive polyps’ and ‘gut polyps’ but otherwise vascular connections and more solid construction may not be too far-fetched. The other option is simply to make them small (I understand this doesn't meet one of your checkpoints). Gas exchange isn’t an issue for insects and frogs.

@Chickensarenotcows mentioned planaria, which have an advantage similar to some worms where they can be completely bifurcated and, as long as a certain portion of the body is left intact, completely regenerate. The way this is achieved in siphonophores and partly in planaria is segmentation - basically modularization of the body. Examples of similarly segmented life (without the regenerative ability of course) are centipedes and worms. A merging of these more complex, terrestrial body plans with the colony zooids/polyps of siphonphores could yield a believable organism of your description (if arthropleura is anything of an example).

P.S. Sorry for the enthusiasm, I just really like these guys.

Answer 2

The problem is that plants were able to evolve the way they did because if you cut them, the only criteria the new limb needs is that it has access to sunlight. This makes the random developmental generation of new material without strong negative consequences.

Doing this for an animal, which requires locomotion, would be extremely difficult. That's why animals like lizards regenerate limbs, but they follow a very specific structure that allows them to walk the same way every time it happens. If they generated a random limb, varying is size and shape, it would make walking very difficult and they would die off very quickly after that. Hence, not very appropriate for evolution.

I propose an entirely new kind of animal with a unique organ or a section of the brain completely devoted to calculating how to use and adapt to its randomly generated limbs. This would allow that animal to have random locomotion patterns, causing each individual of the species to be very difficult to track by predators and thus would benefit the evolution of the species. Some locomotion patterns would be more helpful than others, and as such those patterns would over time be removed from the rules that the generation of new limbs would follow. (ie having limbs of vastly different lengths would be avoided)

This animal would be similar to insects, in the sense that they would not have a circulatory system, because loss of blood could cause death and would inhibit the generation of appendages. Rather, upon losing a section of the body, the animal might go into a hibernation stage dedicated to generating new limbs and training the brain how to use those limbs.

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Summary of why my model works:

1. Be mobile  

My model moves using randomly generated limbs.

2. Be heterotrophic--whether vegetarian or carnivorous doesn't really matter.  

My model works for all kinds of animals, regardless of diet.

3. Be constructed largely out of repeatable, interchangeable, and redundant organ 

complexes, such that damage and regrowth is both possible and expected.  

Other than a lack of circulatory system and a unique organ or dedicated area of the brain, my model does not specify any organs that cannot be regrown.

4. Have its detailed large-scale shape determined in large part by environment and  
injury history, not fixed by genetics.

My model supports its evolutionary development, as well as the ability to generate limbs in a way that can adapt to the environment. For example, developing limbs suitable for climbing, limbs for running, or a combination of both and any other possible uses.

5. Live on land.

My model can be land-based, water-based, or flight-based.

6. Exist in a size range typical of mammals--say, somewhere between a
  housecat and an elephant.

No size restrictions need to be placed on my model. However, the larger the animal, the longer the amount of time limb regeneration would require (leaving the animal vulnerable), which suggests that evolution would tend to lean towards developing the species into smaller creatures.

Answer 3

I think the comments about Echinoderms are actually a good start to get on the right track. It is already the phylum most closely related to the phylum Chordata (which includes mammals), and many of them have similar, if not exact, traits you are describing.

1. Be mobile. No question there. Many species of Echinoderms are mobile, if slow.

2. Be heterotrophic. Again, no issues there, Echinoderms definitely eat stuff.

3. Be constructed such that damage and regrowth is both possible and expected. A notoriously important characteristic of many of the most commonly recognizable Echinoderms.

4. Have its detailed large-scale shape determined in large part by environment and injury history, not fixed by genetics. Here is where we start to diverge from known Echinoderms, and have to borrow from other forms of life. I see 2 main options here, depending on just how mobile the animal is, and just how specialized it's different organ structures are.

Option 1: it is very slow moving, and rarely moves when it can avoid it. It remains in place, foraging any food source in the area immediately within its reach, and stays put until the local food source is entirely exhausted. Whenever possible, it grows toward food it can detect, rather than moving toward it. Its appendages for eating and for locomotion are both very small, very numerous, and interconnected (it eats any food it steps on through its feet). When remaining in its current location is not possible, it uses starfish type locomotion (possibly in conjunction with snail/slug type locomotion) to move to a new food source, and repeats the process. Size/shape of the individual is determined by the shape of the food sources it encounters, as it (at least somewhat) grows in to the shape of that source while it feeds.

Option 2: Slightly faster moving, but still slow. A generally rounded or spherical shape, but with no set numbers of appendages. I imagine something the shape of a sea-urchin, with a random number of spines. More specialized locomotion appendages are present, but other appendages can be recruited for locomotion for faster movement in emergencies. Existing appendages are converted, or new ones grown, as needed, after injury or accident. The key being "as needed". If a missing limb isn't slowing the animal down, or keeping it from feeding normally, etc, no need to regrow it at all. If repeat attacks occur, defensive appendages could be converted to locomotion "permanently", or vice versa, depending on the types of attacks and the results.

5. Live on land. Sea Cucumbers already have a similar external body plan to slugs, and starfish can reach rocks at high tide that are out of the water at lower tides, and stay there until the tide returns, so there is some (little) precedent for out of water (if not fully land based) animals with some of these characteristics.

6. Size range somewhere between a housecat and an elephant. Sunflower sea stars can get about 4 feet across from arm tip to arm tip (eye-balling some google images, it looks to me like just the body, without the arms, is close to 2 feet across), and sea cucumbers can get more than 6 feet long

As far as evolution, it seems to me to be no great leap from tide pools to fully land-based lifestyle, both to evade predators in the tide pools themselves, as well as reach land-based food sources that tide-pool-locked species can't reach, especially if those food sources are similar enough to require relatively small change in digestive function. The first step would probably be water retention, to avoid dehydration on land, followed by specializations for respiratory functions, and then locomotion and food source specialization would be next.

Answer 4

We are already modular, already mobile, just in a different scale

Animals have long had self-repair mechanisms to accommodate loss of cells through either attack/cell death, injury (such as lacerations) or bone fracturing.

It is actually in fact also a simple extension of our growth. As we grow, our cells divide and organs grow. When we are embryos, we do not have all organs yet, but as we slowly accumulate more cells they create more organs as dictated by our DNA.

In fact, analysis of foetal growth is basically a story of how we evolved. There is a reason why we look so 'tadpole' when we are young, yet as we grow more and more features are added.

So what you need is a way for cells to divide and add new organs to replace ones that are lost (perhaps entire new organs) - at a much more drastic, foolproof rate that we do now. This will be difficult but not impossible.

• We need to ensure continuous function, such that absent organs are not missed. We therefore need redundancy in major organs, so we may need more kidneys, hearts and other organs in different areas to provide this redundancy. Such evolution would be a difficult leap, but not unheard of.
• We need the ability for growth to be following a new pattern, in a way which retains functionality. So if our arm is removed, we need to grow nerves, bone, muscle and skin in the same way we grow them as an embryo. This would require coordination of regeneration in a much more smart way than we do currently, but again should be feasible.
• We need a pathway in evolution to achieve this. Evolution is driven by both necessity and sexuality. To accomplish your objectives in your question, there needs to be both a physical need to grow this way, and a psychological desire to do so by your mate. Anything goes, but I think it would be possible to find a route there if these hold true.

Answer 5

I am not entirely Clear on what your question is. I imagine you are asking whether an organism can develop or evolve in such a way that it has the internal systems of a Plant (regeneration and such) as well as the higher functions of animals (locomotion, thought). i.e. something like Dc’s Swamp thing.

This is most likely going to turn into a Biology lecture. And I will be making some conjecture where my expertise is lacking. But, here are my thoughts.

An Organism Capable of locomotion and some manner of self-awareness, does so at sacrifice of other traits, including a modular cell nature. As far as I am aware. Let me Explain.

In RPG terms, i think the biggest challenge for such an organism would be the trade off between High Physical Stats and Processing power. The modular structure of the plants enables them to grow to sizes unattainable by most animals. It facilitates damage reduction and high regeneration such that they can recover being cut down provided the right conditions are met. But this comes at a cost of lack of cognition.

Any animal no matter how small, has a sense of self. They are aware of their own body and have at least the most rudimentary instincts. Even a cockroach or an earth worm has self-preservation instinct. While there are some plants with similar defensive characters, they are more trigger based than instincts. (E.g. Tactile/Odor based irritants & allergens)

The plant body has no centralized structure. Which means any plant cell taken from the whole can perform all the functions of the whole. And this range of functions while impressive are limited in the scope of terms. In Biochemical terms, a cell can perform only so many reactions at a time. Or an interval. If it needs to do more, add more, it must sacrifice some other to make space. It needs more Processing power.

Take this scaled up Macro analogy. Whales were once terrestrial mammals. In time they evolved to be aquatic organisms. And concurrently their physiology adapted to suit their new evolutionary path. Their forelimbs flattened and became flaps, the hind limbs relatively unnecessary for swimming, regressed completely and is now just a vestigial bone making their bodies more streamlined. And the freed up energy, previously spent to development of limbs was re purposed for better respiratory capabilities and lungs that can withstand the strong pressures in the deep waters.

Similarly, in the procession leading from Cell based life forms to Kingdom Animalia, the cells instead of doing everything by itself started to delegate and compartmentalize tasks. In Multi cellular organisms Cells began to Differentiate to accommodate specialized tasks. These specialized cells could do only a fraction of what the originals could. But now instead of a single cell performing n number of tasks, there were x types of cells each performing n/x number of different processes. (An oversimplification. Differentiation in actual physiological systems rarely follow set division of labor)

The remainder of the cell’s available biochemical potential, i.e. its Processing Power could be allotted to new tasks. And believe me you, the capability of self-propelled motion is a game changer. Now, the energy sources available to you are as large as the expanse you can cover on your own. This albeit means that you’ve sacrificed the capability to utilize some of the micro nutrients and minerals.

With Locomotion comes more specialization. Because remember, now you are actively interacting with your environment as opposed to be molded by it. And each specialization develops it own sub-specialization.

All this necessitates the need for a centralized control scheme. Because the old way of moving by ‘sensing’ the higher concentration of nutrients is not always viable over the now (relatively) large distances. Which necessitates more processing power, which needs more energy and more specialization. And so, we ascend further in the ladder of complexity.

And Specialization is the death of Flexibility. In the current scheme of complex organisms all cells in the body have, in theory, the capability to perform all necessary physiological functions. But not at the same time. These progenitors, (Stem cells) become differentiated to serve their assigned role as early as day 5 of development. (Note that we’ve shifted from talking about Asexual cloning methods, to gene mixing in form of sexual fertilization). From then on, until the death of that organism individual groups of cells performs only the function assigned to them.

And as mentioned before, this system is so welded in place that such groups of cells are virtually irreplaceable. Except the Brain. Which is ‘literally’ Irreplaceable. The Brain, one of the crown Jewels of the Animal Kingdom, and the greatest energy sink in the body (20 percent of total available) is so vital, stopping its function for more than a couple minutes would end that organism’s existence. Even when all other organs are working perfectly. And it’s so individualistic replacing it with a spare is, let’s say is NOT the preferred fix. The condition called Brain death results in a body which in theory is in working condition; and hence one of the reasons that victims of brain death are treasure troves for multiple organ replacements.

To Summarize: The very reason that you are ‘aware’ of your existence means you’ve given up the chance for the unbelievably broken ability to regenerate from the smallest of parts. While the lack of ‘awareness’ plants and other cellular entities capable of such feats are not in a position to, let’s say ‘appreciate’ it.

Back to the game analogy. You are a mage that sacrificed physical vitality to gain intelligence for casting spells. While the other side is a passive Berserker/Tank that gained near infinite regenerative capabilities but are nothing more than collected mass. (Implying plants are dumb, is an incorrect and highly inaccurate statement. And in some circles, intelligence is not measured in same standards to plants as they are to animals. But you get the idea.)

P.S. I’m aware that some parts of my answer have run far, far off tracks. But if I was able to assist it in any ways I’ll feel satisfied. As was this a good thought piece for me.

Answer 6

A modular animal could be similar to an lizard in overall shape, but with the legs and tail branching out modularly like a plant. This creature would also have to be herbivorous, have extremely good predator defences, and have very little competion, as these traits would mean that they never need to move quickly, and reducing the selective pressure to become more standardized

Another way modular animals could evolve is if a sessile modular animal, due to an extinction event or something similar, ended up as the only moving organism in their area, which would likely lead to them evolving to become motile again

Answer 7

Interestingly enough, afaik your chosen terminology for the question might've been de-railing.

"Plant-likes" are more similar to a Reaction-Diffusion System than modular Legos (it's how leaves, roots, stomata, etc. are "located" and then grown). "Blob-likes" even more so. Whereas "Fixed-likes" are more like modular Legos, where each has been pruned down to the bare minimum and essential arrangement.

By necessity your looking somewhere on the spectrum of Chemical Soup for a single organism: from Chaos (a la Calico Spots) to Order (a la Colonies) with as little specialization as possible. Injecting modular specialization Legos here and there adds spice to the creature but also weakness, unless it's a redundant or "omnipotent" piece. In which case you're leaning more towards Order anyways. A cat for example has many Legos: specialized cells modularized into skin, hair, and other pieces. If you averaged cells' competencies and merged them into a couple few cells to increase redundancy; you'd have very little structure left besides diffusion-reaction, diffusion-aggregate, or crystalline so your body plans would be limited. A knee for example requires too much coordination, imagine growing any plant and hoping it formed vague muscle shapes and separate branches coming together in a socket-like shape! Even the absolute squinty-eyed rudest approximation would be sensational. On the other hand......... Your best bet for what you want is probably (as has been mentioned):

Symbiosis

Just make a bunch of little interchangeable symbiotic creatures. This one does thinking, this one does acid, if they're essentially "specialized cells" but can survive alone, just better together... You've got a modular creature with very few constraints! (And as @XenoDwarf's answer pointed out: they don't even necessarily need to be able to survive solo. But that limits you a little bit, even if you gain some efficiency)

You've probably got a higher caloric requirement with symbiosis, which if you're familiar with integration or inlining in any of various of contexts (programming, economics, etc.) will probably be pretty easy to see. But other than that issue it's probably the best fit!

Answer 8

A tapeworm fits your description.

Tapeworms are intestinal parasites of land and water vertebrates. A tapeworm has no mouth, intestine, excretory organs or respiratory organs. The worm's body consists of the scolex, which is an organ it uses to attach itself to the inside of the hosts intestine. After that it is repeated segments which are your modules. Each of these segments (proglottids) contains sex organs with which it can reproduce. The entire worm comprised of many modular segments can move about in the host's intestine. Proglottid segments are motile too. Each proglottid segment can break off and form a new worm.

A worm will have only one scolex, I think. Unless it breaks in half in which case the one without the scolex will grow one.

The worm is simple and that is how it can get away with this modular plan. As regards a body plan dictated by circumstance and history, in the case of the various tapeworms that adds up to length. Length is determined by how many modular repeats (proglottids) the body is made of.

The length of the worm will vary and this is where it meets your last criterion, for size. Your low end is the size of a cat. These worms can grow to be 20 meters long. I assert that is a cat's worth of worm meat.