Lying just under the gills, and side by side with them, is a smaller, plume-like organ having the appearance of a smaller gill; it is called the /osphradium/. The office of this organ is not definitely known, but the supposition is that it is the seat of the olfactory nerves. The osphradia are not always present, but whenever found they are placed in close conjunction with the gills, in order, probably, that the animal may best determine the quality of the water brought by the siphon to the breathing-organs. In /Buccinum/ the osphradium is especially prominent. In all mollusks the sense of smell is highly developed. A dead animal anchored to the bottom will very soon attract the carnivorous /Mollusca/ from every direction. This is not a bad way to trap certain species that live upon rocky stations and are otherwise difficult to secure by the dredge. On the left-hand side (right of the animal), and adhering to the under side of the mantle flap, which has been thrown back, a somewhat inflated and convoluted tube, the rectum, will be seen. It terminates in an opening just within the mantle-edge. By the side of this are the renal and genital openings. * * * * * Having now examined the external organs, we may investigate the internal anatomy of our subject. To do this thoroughly requires considerable skill, but for the purpose of merely discovering the relative positions of the principal internal organs a little care only is necessary. The work will be greatly facilitated by boiling the animal for a few minutes, for this serves to harden the tissues and to separate the various organs in a most satisfactory manner. To begin, open the proboscis from the mouth, cutting back so as to expose the esophagus. It will be seen that the throat is long and very muscular. [pg340] RADULA Hidden among the red stripes of muscular fiber which line the inner surface of the throat will be found a fleshy strip covered upon one side with many transverse rows of chitinous teeth. This is the /lingual ribbon/, or /radula/. Remove as much of it as possible, and examine it with a hand-glass. The radula is a most important organ, and is a unique possession of /Mollusca/, except the pelecypods, or bivalves. Usually it lies coiled up like a spiral when not in use; its purpose is to tear and rasp food. [Illustration: Three rows of teeth from the radula of /Fasciolaria trapezium/.] [Illustration: Portion of the radula of /Imbricaria marmorata/.]