<p>Doridoidean nudibranch mollusks integrate into their integumentary system, thus, into their body-wall tissue, large quantities of Ca-carbonate spicules. We report the chemical and structural characterization of the body-wall spicules of the sea slug <i>Baptodoris cinnabarina</i> (Discodorididae, Doridida). The aim of the study was an in-depth characterization of the spicule mineral and of the soft tissue within the spicules. We achieved the latter by using a combination of imaging (TEM, SEM, micro-CT), diffraction (XRD, EBSD, SAED) and spectroscopical (ssNMR, EDS) techniques. We demonstrate that the spicule mineral consists of amorphous calcium carbonate (ACC), complemented with amorphous calcium phosphate (ACP). The amorphous minerals are very stable and do not crystallize, even not at prolonged electron beam irradiation. We distinguish structural and chemical differences between the rim and the core portions of the spicules. The spicule rim consists of an array of concentric organic laminae and is reinforced with ACC and ACP granules. At the spicule rim the organic laminae are closely positioned, while at the spicule core the laminae are widely spaced. The spicule rim is strongly enriched in phosphorous and has increased magnesium contents, while the spicule core is highly enriched in magnesium and has low phosphorous contents. We observe in the spicules a rather graded distribution of ACP, ACC, organic substance, phosphate and magnesium. We propose that magnesium, organic substance and the phosphorous of the ACP stabilize spicule ACC, both within the living organism as well as when the spicules are extracted from the soft tissue and are kept separate.</p>