The osphradium is an olfactory organ in certain molluscs, linked with the respiration organ. The main function of this organ is thought to be to test incoming water for silt and possible food particles.
The organ is present in all members of the genus Conus, the cone snails, a group of predatory sea snails.
It is a popular idea among malacologists that the presence of an osphradium should be a molluscan synapomorphy. However, a truly homologous "osphradium" is yet to be found in some groups, such as in Lepidopleuridan chitons (Polyplacophora).
References
Osphradium Wikipedia(Text) CC BY-SA