Forams are also members of the sarcodina group. They start life with a one-chambered shell, and add compartments as they grow. Their pseudopodia project out through holes in the shell. They feed on diatoms and other protozoans by secreting digestive juices into their food to dissolve it. That places them at the very base of the food web.
Many animals eat plankton directly, or feed on animals that eat plankton.