The Platyhelminthes (or flatworms) might be what you're looking for. They have no circulatory system, instead relying on diffusion through their skin and tissues to obtain the oxygen they need and expel waste CO2. They also have no respiratory organs, nor blood, lymph, or any other body fluid...the simple flatness of their bodies is sufficient to ensure that oxygen can reach their innermost cells. But they do have a primitive brain, a two-lobed cortex of nerve cells and a core of nerve fibers that take in sensory information and coordinate movement for the rest of the body.

[deleted] t1_ja2hqoy wrote on February 26, 2023 at 10:09 AM

#2,019,806

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[deleted] t1_ja2p9di wrote on February 26, 2023 at 11:50 AM

#2,020,942

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[deleted] t1_ja584qd wrote on February 26, 2023 at 10:55 PM

#2,037,828

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Blank_bill t1_ja5axvf wrote on February 26, 2023 at 11:14 PM

#2,038,354

Replying to djublonskopf (#2,018,921)

Amazing, I wouldn't have thought that they had even that complex a brain.