A comparative histological study was undertaken to reveal the morphological diversity and systematic characters of a radula-supporting organ of gastropods. Observations on 33 species, all from different families, revealed six major morphological characters: (1) the number of odontophoral cartilages or radular bolsters: 0, 1 (fused), 2, 4, 5, 6 and 10; (2) histology categorized into 6 types based on the properties of cartilage matrix and cells; (3) the presence or absence of an enclosing membrane of the cartilages or radular bolsters; (4) the presence or absence of overlapping of the right and left cartilages or radular bolsters; (5) the closest position of the cartilages or radular bolsters to each other in cross section at ventral or dorsal side; and (6) the insertion areas of the ventral approximator muscle connecting the cartilages or radular bolsters – ventral, medial, or outer lateral area. Outgroup and ingroup comparisons based on recent phylogenetic hypotheses suggest the following evolutionary scenario for gastropod radula-supporting organs: the ancestral gastropod is assumed to have possessed two pairs of odontophoral cartilages with a thick matrix and ventrally connected by the approximator muscle. The cartilages have possibly independently increased in number in patellogastropods and Neritimorpha, decreased into a one pair, single piece or lost in Caenogastropoda, and replaced by connective tissue and muscle fibers in Heterobranchia. Some taxa such as Cypraeidae have gained a unique histology. The cartilages or radular bolsters are closest ventrally in cross section in the majority of gastropods but closest dorsally in part of the taenioglossate Caenogastropoda. The diversification of these character states in gastropods seems to be phylogenetically constrained, not ecologically.