The internal structure of trilobites is revealed using the X-ray stereograph. The alimentary tract (the gut) was housed in the axial lobes, and the mouth is behind the hypostome. There is an array of external gills placed under the pleural lobes. The pleural rings protected both the gills and legs. The external gills are used to exchange gas between the internal body and the environment.

The upper picture is a cross-sectional view from the front and the lower picture is cross-sectional view from the side. Both pictures are modified from (Prothera, 1998).