Most free - living nematodes are
microscopic, though a
few parasitic forms can grow to several meters in length (typically as
parasites of very large animals such as whales).
We have small
parasites that are
microscopic and can only be seen under a microscope with specific stains: Giardia, Entamoeba histolytica, Blastocystis hominis, Cryptosporidium, Dientamoeba fragilis, Entamoeba coli, Yersinia enterocolitica, Entamoeba hartmanni, and Toxoplasma to name only a
few.