There are three levels of organization found in all animals. Cellular, tissue, and organ. Protists are single celled organisms containing organelles. Heterotrophic nutrition in most protozoa vs. autotrophic in most algae.
Autotrophic organisms create their own food.
Heterotrophic organisms must hunt for their food. Mechanisms of osmoregulation, excretion and gaseous exchange. Gas exchange and excretion in cellular organisms occurs through the plasma membrane or cell wall. In freshwater protists, osmoregulation is taken care of by contractile vacuoles which absorb excess water in the cell and then excrete the water back into the environment. Osmoregulation is less important in saltwater protists.