Exceptionally well-preserved examples of the calcareous annelid tube Ditrupa bartonensis occur within clays of the Eocene Barton Beds Formation of Hampshire, UK, where it is the most common of four recorded annelid species (D. bartonensis, Protula extensa, Pyrgopolon (Serpula) crassa, Pyrgopolon (Sclerostyla) mellevillei). The current work confirms that i) chevron-shaped lamellae are present within Ditrupa, and that ii) all microstructures in Ditrupa are conservative (Eocene to present) - including the inner tube structure, which has only been previously recorded from extant material. New observations of the regularly ridged prismatic (RRP) structure of the outer layer illustrate the occurrence of conchoidal fractures, which are likely to represent the influence of acidic proteins within their calcite crystalline structure. The occurrence of conchoidal fractures, and the intricate non-crystalline external appearance of RRP structure, suggests a similar mode of formation to that of the single calcite crystals that form the plates and spines within echinoderms, and likely reflect an adaptation for tube strengthening, as previously postulated. A tentative model for RRP construction based on the formation of echinoderm single calcite crystals is suggested with the aim of stimulating further research. The inner layer, which comprises partially aligned slender prismatic crystals, was previously referred to as irregularly orientated prismatic (IOP) fabric, but does not fit the original description. Therefore, simple preferentially orientated prismatic (SPOP) is introduced for such fabric that forms the inner layer of Ditrupa.