The anatomical basis of function in the buccal mass of prosobranch and amphineuran molluscs

A comparative account is given of the muscular anatomy of the buccal mass in chitons and a number of species of prosobranch gastropods. From this can be deduced a basic muscular organization consisting of buccal constrictors and dilators, protractors and retractors of the odontophore, protractors and retractors of the subradular membrane and an approximator muscle of the supporting cartilages. In addition to these many ancillary muscles act as tensors: these are numerous in chitons, monoplacophorans and rhipidoglossan proso‐branchs. In the first two groups, which have a poorly developed anterior end covered by a shell, they act in place of the muscles of the mobile wall of the head of gastropods; in the last they are necessary for the fine control of an odontophore brushing the substratum. In prosobranchs which forcibly rasp their food they are lost. The dominating influence in the evolution of the buccal mass in neogastropods is the proboscis. Its narrowness and elongation have had profound effects on the anatomy of the buccal mass.