The Protozoans are acellular animals without tissues or organs. They exist singly or in colonies. In these colonies all cells are alike. But in "Proterospongia" a colonial flagellate some degree of differentiation exists among the individuals forming the colony.
- Contractile Vacuole" for water regulation in the body.
- Photoreceptors" as sensory structures.
- Cilia and flagella" for locomotion and food collection.
- Skeletal secretions for protection and to give shape.