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Spiraculate blastoids have extraordinary internal skeletonized respiratory structures, the hydrospires. However, the detailed pattern of seawater flow within them is unknown, making it difficult to assess their respiratory effectiveness. Using a scaled-up (72x) 3D printed physical model to visualize the flow of water through the most distal (aboral) part of the hydrospire of Pentremites rusticus, we show that flow was consistent with effective respiratory exchange in the hydrospire folds – the flow continued horizontally within the hydrospire folds after passing through the hydrospire pore canals and only developed an adoral component to its velocity once it had entered the hydrospire canals. The observed orderly laminar flow is consistent with the Reynolds numbers we estimate for a living blastoid (Re = 0.0008–0.05). While most functional analyses of spiraculate hydrospires focus on their respiratory function, it is also possible that they played a role in feeding, helping to draw water past the brachioles, which is a hypothesis that is amenable to future testing. Tony L. Huynh. Department of Integrative Biology, University of California, Berkeley, Berkeley, California 94720-3140 USA. huynhtony@berkeley.edu Dennis Evangelista. Department of Biology, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3280 USA. devangel@live.unc.edu Charles R. Marshall*. University of California Museum of Paleontology and Department of Integrative Biology University of California, Berkeley, Berkeley, California, 94720-4780 USA. crmarshall@berkeley.edu *Author for correspondence

Visualizing the fluid flow through the complex skeletonized respiratory structures of a blastoid echinoderm