The other nite I went to bed with a good book, the 2nd edition of British Prosobranch Molluscs by Fretter & Graham, and read about the siphons of marine snails.
The siphon is a tubular formation arising from the edge of the mantle. The snail extends it out of its shell to suck in water, which flows past the gill (ctenidium) in the left side of the mantle cavity, exchanges its oxygen with carbon dioxide, and then comes out the right side. The drawing below shows the direction of the water current within the mantle cavity of a typical marine snail.
Arrows indicate the directions of the water currents in the mantle cavity of the marine snail Nassarius. (From Abbott, American Seashells, 1954.)
A week ago I ran into several individuals of the marine gastropod Melongena corona on a sandy bottom in ankle-deep water near Tampa, Florida. The largest snail became the subject of my photographs.
When viewed from above, Melongena's siphon appears to be a tube, but it is actually open along its bottom. The close-up of the underside of the siphon reveals its anatomy.
According to Fretter & Graham, the siphon is an elongated extension of the edge of the mantle that is rolled into a tube. The drawing below explains this.
Partially unrolled siphon (si) of the marine gastropod Buccanum undatum. ct, ctenidium; me, mantle edge. (From Fretter & Graham, 1994).