The parasites are widely distributed in many species of the phylum Gastropoda and Gastropoda, but they are most prominent in shellfish that feed on plankton. It is collected in a rod capsule, the anterior end of which extends into the stomach and comes into contact with the gastricshield on the stomach wall. The enzymes contained therein are slowly released into the digestive juices as they rotate and grind against the gastricshield in the living organism. This is optimal for digestion of the small amount of plankton ingested at any one time, and it is thought that the oyster rotates about 70-80 times per minute. Some of the crystal rods are soluble and disappear when the shell is removed from the water (oysters); some are solid and insoluble (clams, horse oysters). The enzyme contained in the amylase activity is the strongest, in addition to weak cellulase, oxidase and esterase. No other glycolytic enzymes, lipases and proteases.