Many cryptobranch dorid nudibranchs contain innumerable calcareous spicules, yet the function of these elements is unknown. Two possible roles are defense against predators and structural support. In one dorid, Cadlina luteomarginata, whole‐mount and thin‐section staining revealed an intricate network of spicule tracts and connective tissue ramifying throughout the body, with muscle fibers associated with this spicule/connective tissue matrix and inserting into it. Spicules were present in high concentrations in all areas of the body, but highest in exterior mantle tissue. Relative investment increased isometrically with body size for most body regions, in contrast to the positively allometric investment seen in prosobranch shells. Bioassays with artificial food indicated that spicules alone did not deter generalist crabs and anemones, and only weakly increased the deterrence of secondary chemicals to anemones. Thus, while nudibranch spicules may serve as a defense against other predators, their primary role may be in body support.