Motile v. Sessile Pathways in the Transition Between Microscopic & Macroscopic

[Rathalos]

@MirrorMonkey2

The answer I have in mind is illustrated by flatworms. They can be up to 9cm long, but their body is extremely thin, 1-2mm at most, because they don‘t have a circulatory system and rely on diffusion for gas exchange.

I think you are mostly correct, but I think beyond just not having what can be strictly described as circulatory system they don’t have a body cavity at all. By which I mean a fluid-filled space inside the body that is not made of cells/fluid, that oxygen and other things can diffuse through, even without explicit “pumping”.
(And what in my opinion also makes a difference is that this means a significant amount of the volume of the animal is not living cells, so not consuming resources. Jellyfish, sponges, etc. can get big, but have two layers of cells usually only a single-cell thick, sandwiching non-cell jelly.)

This is also a reason why if possible I would to have non-cell parts or cavities in Multicellular, so we can represent this at the scale where you’re thinking about cells. But whether you leave Multicellular without a cavity, or the Stage does not have that feature at all, I do agree that you should have to develop one in order to grow much in size (without being increasingly flat) in Macroscopic.

@MirrorMonkey2

For this reason I suggest that the player starts the macroscopic stage with an organism that is 1cm long, but qiet flat (~1mm).

Mostly agree except for if we include the ability to create cavities in Multicellular. Species that have those could start out a bit more thickened.
We could also of course assume you develop a body cavity in the transition to Macroscopic, but I think that’s a missed opportunity.

@MirrorMonkey2

I‘m not sure what comes first in this cascade of developments. Do muscles evolve which then necessitate dedicated respiration? Or does respiration evolve which then im turn allows gas-exchange-heavy muscles to evolve? Either way it feels like these are somehow tied together and are necessities for mobile organisms whose thickness crosses the threshold beyond which diffusion alone is no longer viable.

I don’t think muscles and respiration are quite this linked, and muscles quite clearly pre-date specific respiration structures. Various tiny microscopic animals have muscle cells, and simple animals like jellyfish have them. This is also why they are on the Multicellular Stage roadmap. The flatworms you mentioned before also have muscles.
Respiration is in my opinion moreso linked with increased thickness and increasing scale in general, compounded by increasing resource consumption, like muscles of increasing size.

@MirrorMonkey2

making this flat starting organism able to move without muscles, relying solely on organelles like flagella and cilia for locomotion (which aren‘t be visible at this stage).

Like I think I mentioned earlier in the thread, Indo agree that cilia and flagella should have value here, but I think muscles should be the winner in most cases.
As for being invisible: the cilia on Ctenophores can be up to 2mm long, so I feel like they can at the least affect the visible texture?