The most difficult organ to observe is one that is only found in four species of bivalves, the crystalline style . It may be present in oysters that are actively feeding. The style is unique in that it rotates at the anterior end against the gastric shield. This rotating organ helps grind the food as well as mix the food with enzymes which it produces. It accomplishes this within the digestive gland where digestion and assimilation occur.
Phagocytes are present within the tubules of the digestive gland which empty into the stomach.
Within a short time of removing oysters from the water the crystalline style decomposes, so careful observation in fresh specimens is necessary to observe this organ.