The Gastric Mills of Crayfish and Other Crustaceans
"Chew All Your Food Dear....With Your Stomach!"
It's not often that you can wax poetic about the chewing, grinding, and digesting of invertebrates, but here goes. Many crustaceans are highly specialized for their aquatic lifestyle. Inside their digestive system they have a unique stomach that is called the GASTRIC MILL. The gastric mill is found in crabs, lobsters, crayfish, barnacles, krill, and many others. These invertebrates don't have teeth in their mouth to grind their food, so they process it a bit differently. Their claws rip and tear apart their food (mostly plants and animals), their mandibles (or mouth parts) shred the food down a bit more, and then it is passed on to the digestive tract. The gastric mill functions a bit like a gizzard in a bird, but unlike a gizzard which has rocks and sand for grinding the food, the gastric mill has strong muscles which are folded into ridges that increases surface area for absorption and helps in mechanical breakdown of the food.
Depending on the species of crustacean some have ossicles or calcified plate-like structures in the stomach (much like the flat surfaces of molars) while others have chitenous gastric teeth. Chiten is the same material that the arthropods shells are made out of, and is very strong. In some cases, after going through the gut, food particles may still be too large or hard to break down, then the particles may be ejected back out of the mouth and reprocessed. In many crustaceans, there is a set of glands within the stomach to help with digestive absorption and secretion.
In crayfish that are preparing to molt, you will find two structures in the gastric stomach called gastroliths. These are literally stashes of chiten (shell building material made of calcium carbonate) that the crayfish calls upon after it sheds it shell. During the molting process the old shell is cast off, strong juices flood the muscular stomach and break down the gastroliths dispersing the calcium carbonate to the body for building back the crayfish's shell. Only fresh-water invertebrates need gastroliths because calcium carbonate is not as readily available in fresh water as it is in sea water. (NOTE: the gastroliths in dinosaurs were rocks used for digestion, much like birds, but the gastroliths in crustaceans are for exoskeleton development and not solely for digestion).
I can tell you, it's quite disconcerting for first year biology students to open up a crayfish and find teeth in the stomach! They often ask if the crayfish swallowed a fish or animal with teeth. These teeth are strange, but they play a very important evolutionary role in processing food. If you get a piece of food stuck in your teeth, just think how it must feel to a crayfish. I can only imagine the belly-dancing moves they would have to make to get the food unstuck...and flossing is quite out of the question!
Here are some teaching links for crayfish dissection: