3D reconstructions of the gastric caecum of selected irregular sea urchin species. The gastric caecum is a translucent body free of sediment and probably constitutes one of the main sites of digestion [38, 39]. The upper diagrams show an aboral view, the lower diagrams a lateral view with the anterior side (ambulacrum III) oriented towards the right-hand side. Arrows indicate the position of the junction of the gastric caecum with the stomach. (A) Echinoneus cyclostomus, Echinoneoida. (B) Echinolampas depressa, Cassiduloida. Species of this sea urchin taxon presumably all possess a highly reduced gastric caecum consisting of numerous small blindly ending sacs. (C) Pourtalesia wandeli, Holasteroida. (D) Abatus cavernosus, Spatangoida. Scale bar: 0.5 cm.