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This question will be highly related with this other How could an efficient respiratory system evolve for giant arthropods?, because both systems do an important and combined contribution to oxygenation of the animals tissues.

The circulatory system of arthropods is open, that is, there is no closed circuit of vessels through which a differentiated fluid circulates, which properly could be called blood, arthropods don't have specialized cells to transport oxygen, and the liquid which transport nutrients and oxygen is called hemolynpha. The hemolynpha is moved throughout the body by different muscles which push that till disembogue to each organ.

The two closest groups to arthropods that could evolve a closed circulatory system are anelids or worms and cephalopods, so can be possible that a common ancestor with no closed circulatory system has developed one.

enter image description here

So, from the current structures that the different arthropods have, how they could evolve to meet the requirements of a larger creature?

Basically this question is searching for the series of mechanisms and process that must evolve and appear and how they would work to make it possible.

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  • $\begingroup$ Is ‘evolve into a closed system’ a valid answer or do you want the end result to still be open to some degree? $\endgroup$
    – Joe Bloggs
    Aug 8 at 5:26
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    $\begingroup$ @John "shedding the exoskeleton limits size just as much" // you're talking about the heavy loss of investment & period of vulnerability while the new one hardens each time it's shed right? plus of course something of 45 kilos with suddenly no skeleton (endo or exo) during the moulting stage would likely die through their organs being crushed by it's own weight in similar fashion to a beached whale? $\endgroup$
    – Pelinore
    Aug 9 at 13:31
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    $\begingroup$ ^ @John those issues could perhaps be fixed by segmental moulting rather than shedding the entire exoskeleton in one go or rather than shedding at all periodically disarticulating the plates when they'd normally moult & hardening new exoskeleton on the edge of each plate to fill the gaps produced by the bodies expansion? you'd be able to age them by the growth rings on the shell segments same as you can a tortoise then :) $\endgroup$
    – Pelinore
    Aug 9 at 13:43
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    $\begingroup$ sectional molting may help, but then the animal spends a long time helpless, the latter idea will not work because of the complex geometry of the exoskeleton, its not like a bone were you dissolve part of it while growing other parts to change the shape. you also run into the structural limits of a skeleton grown from a single contiguous surface, you end unable to reinforce the joints because you can't make joints surfaces very thick. the joint surface of a human knee is several square inches, an exoskeleton can't make a surface even a fraction of that. $\endgroup$
    – John
    Aug 9 at 16:50
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    $\begingroup$ @John assuming all the other issues like oxygen transport can be fixed then one way or another being a long time helpless (or at least partially crippled with this or that leg largely unusable etc) is probably just the unavoidable price you'll have pay for getting really big with an exoskeleton .. which might be a good excuse for cooperative group instincts to evolve in the species, hive insets would have a leg up on that. $\endgroup$
    – Pelinore
    Aug 10 at 17:19
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Already exists

Coconut crabs and other land hermit crabs have a closed circulatory system and breath air, because they are an island species it is hard to pin down just when they evolved this feature but it has to be less than 98 million years. Likely it is less but as I said it is difficult to pin down since we don't have any intermediaries.

crabs already have a very complex and extensive circulatory system for distributing and oxygenating blood, they lack an efficient system to collect it, relying on the impermeability of the body wall. robber (coconut) crabs have a passive network for reclaiming it as well, similar to our own lymph system, they just are not connected. Connecting them is a rather tricky step, our own body needs two reclamation systems (veins and lymph) because the system has to leak to function, but with high pressure you need to reclaim in quickly. So we reclaim most of it quickly and recollect the leaked portion more slowly in a passive system.

They also use a branchiostegal lung system which evolved from gills, the best way to describe it is imagine lungs that are inside out and inside a larger cavity, instead of the lungs being the cavity.

https://bmcecolevol.biomedcentral.com/articles/10.1186/1471-2148-13-128

https://onlinelibrary.wiley.com/doi/abs/10.1111/maec.12369

https://www.sciencedirect.com/science/article/abs/pii/0034568773900297

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https://www.researchgate.net/publication/229323552_The_morphology_and_vasculature_of_the_respiratory_organs_of_terrestrial_hermit_crabs_Coenobita_and_Birgus_Gills_branchiostegal_lungs_and_abdominal_lungs

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  • $\begingroup$ The second source doesn't work. But for what I found, yeah, looks like the anomura crabs are that apparent middle step between being open and then closed. I hoped you could elaborate a little more on how it works and how it happens, unless that information is in the second source $\endgroup$
    – Drakio-X
    Aug 9 at 0:48
  • $\begingroup$ @Drakio-X should be fixed, As I mentioned how is tricky since we don't have any intermediaries $\endgroup$
    – John
    Aug 9 at 2:15
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    $\begingroup$ @Drakio-X should be fixed, As I mentioned how is tricky since we don't have any intermediaries, the last two sources are a detailed breakdown of their cardio vascular and respiratory system. $\endgroup$
    – John
    Aug 9 at 2:21

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