Overview chart showing analyzed specimens of 'regular' sea urchins and corresponding 3D reconstructions of selected internal organs. (A) Information on species name, geographic distribution, and systematics. (B) Photograph of scanned specimen, aboral view. (C)-(E) 3D models of reconstructed selected internal organs, stepwise turned by 90°: (C) aboral view (interambulacrum 5 facing upwards); (D) lateral view (interambulacrum 5 at back); (E) oral view (interambulacrum 5 facing downwards). The buccal sacs of Caenopedina mirabilis as well as the siphon of Stomopneustes variolaris could not be seen on the magnetic resonance imaging sections. Scale bar: 1 cm. The colour legend specifies organ designation.