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> The strong peristaltic waves of its guts pushes its blood around, since its heart is weak beating:
> 
> https://cosmosmagazine.com/biology/weak-hearted-sea-spiders-use-guts-to-pump-blood
> 
> "Most animals use the strong, steady beating of their hearts to move blood around their bodies and keep up the supplies of oxygen to the tissue that needs it. Not sea spiders: they pump their guts instead.
> 
> "While the underwater arthropods do have hearts, they beat only weakly. This discovery, made by H. Arthur Woods of the University of Montana after spending “a lot of time just watching blood and gut flows in sea spiders” while stationed at McMurdo Station in Antarctica, led Woods to realise that the heart was only circulating blood in the small inner section of the creatures’ legs.
> 
> "He had also noticed that the guts of the sea spider, which are complex and many-branched, extending down to the end of each leg, underwent frequent waves of peristaltic contraction much stronger than digestion would require.
>  
> "After conducting a series of experiments and observations in 12 sea spider species, which involved video microscopy of tracers in the animals' hemolymph and guts together with experimental manipulation of the guts' ability to contract, Woods and his colleagues were able to confirm that the guts were in fact doing the heavy lifting of moving the blood around.
> 
> "Strange as it may seem, the finding highlights how evolution often finds multiple ways to solve the same problem."
> 
> Comment: Lobsters have no heart, and muscle movement sloshes around the whitish fluid that is their substitute for blood.