I am fascinated how most snails (mollusks) scrape algae off of surfaces to eat. Here is my Wavy Top Turban cleaning growth off the side of my tank.
It eats with it's radulla, which is made up of a fibrinogenous (firm) material called chitin. Chitin also forms the exoskeletons, or outside shells of animals like lobsters and crabs. A cool fact: Chitin is made from polysaccharide chains, which is chains of monosaccharide sugar molecules like the sugar you put on your cereal. (poly =many, mono=one). Sooo, does that mean that if you lick a crab's shell, does it taste sweet? No. Polysaccharides are not as sweet because they do not readily bind to the sweet-receptors on our tongue, as the other smaller monosaccharides ones do! Their larger size just does not fit properly on the sweet receptors on the tongue, so does not trigger any response. (https://chemistry.stackexchange.com/questions/88796/why-are-polysaccharides-not-sweet-in-taste)
Below is a cool scanning electron microscope image of a snail radulla.
N.lamellose radula - Scanning electron microscope image by Carrie Tyler, Miami University, and James Schiffbauer, University of Missouri)
Here are some very good diagrams which simplify the process. The radulla starts out in a sac that protects the body from the teeth (a good thing). The radulla slides forwards over a piece of cartilage called the odontophore, which is ALSO moving forwards at the same time. The radulla scrapes off food using the teeth, then brings the food to the mouth as it returns into the sac. What causes all this to happen is a coordinated movement of two muscles that pull the cartilage back and forth, and one muscle that pulls the radula forwards.
Now watch the video again. Although the image in the video is small, you can just see the rows of teeth flipping by as they slide forwards and down. The mouth is closed as the radulla moves back along with the scraped off algae.
Another cool fact: Sea Urchins scrape algae such as kelp with teeth as well. They do not have a radulla, but rather 5 teeth arranged in a circle. Also, there is iron incorporated in the teeth. Iron is a LOT harder than chitin, and so when THEY feed on the plastic sides of my tank they leave scratch marks! As long as I keep them well fed with kelp from the seashore it is not a problem.
For an even closer up image of the radulla see: https://www.sciencephoto.com/media/479570/view , or just google snail radulla and look at Images. You will see a whole fascinating gallery of radulla from many different animals. (many copyright protected so not pictured here)
Is this mouth the inspiration for "Alien"?