Experimental animals were fluorescently labeled in one of three ways. Proteins and dextran are water soluble, and can enter the cell only by pinocytosis or receptor-mediated endocytosis. The RITC label remains intact and functional even within the cytoplasm. Rhodamine fluoresces at red wavelengths. This size particle can only enter cells by phagocytosis.
Acridine orange is hydrophobic in neutral pH, but is protonated in acidic conditions.
Protonation makes it cationic, which prevents it from passing through the hydrophobic layer of membranes. As a consequence, it enters membranous organelles easily, but if the contents of an organelle is acidic, it will be trapped within the organelle. Since lysosomes are acidic, membrane-bound organelles, acridine orange serves as a fluorescent label specific for lysosomes. Working under a dissecting microscope, lower the coverslip slightly by touching a lab tissue to the edge of the coverslip, until the worms just stop moving. Check them under your phase contrast microscope to make sure the coverslip has not killed the worms.