osphradium

from The Century Dictionary.

• noun The so-called olfactory organ of mollusks; a patch or tract of specially modified epithelium of the body-wall at the base of the ctenidium, supplied with a special nerve, supposed to smell, taste, or otherwise test the water which the animal breathes, thus functioning as a special sense-organ.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Zoöl.) The olfactory organ of some Mollusca. It is connected with the organ of respiration.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun zoology The olfactory organ in certain molluscs, linked with the respiration organ. Its main function is to test incoming water for silt and other possible food particles.