Before becoming full worms, Sandworms began their lives as sand plankton, then matured into the sandtrout form. In that phase the swam freely through the sand and sealed off all available water in the porous lower strata.
Fremen/DE children played with sandtrout by placing them on their hands and amused watching them mold themselves to the shape; they would then shake the trout off and admire the "gloves" thus formed.
Pardot Kynes/DE was worried that these "water-stealers" could dry out any area his Fremen/DE chose to plant. He captured several then loosed them in one of the Biological Testing Station gardens. The plant roots prevented them to perform their function and those who did not leave just died out.
Inspired by the Fremen game, shortly before his assault against his aunt Regent Alia Atreides/DE, Leto II/DE placed sandtrout over his entire body (allowing open space only for his mouth and nose) and with the sandtrout skin he managed to defeat her. He then transformed to a sandworm entity.