Protozoa are a very varied group of single-celled organisms, with more than 50,000 different types represented. The vast majority are microscopic, many measuring less than 1/200 mm, but some, such as the freshwater Spirostomun, may reach 0.17 in (3 mm) in length, large enough to enable it to be seen with the naked eye. Scientists have even discovered some fossil specimens which measured 0.78 in (20 mm) in diameter. Whatever the size, however, protozoans are well-known for their diversity and the fact that they have evolved under so many different conditions. One of the basic requirements of all protozoans is the presence of water, but within this limitation they may live in the sea, in rivers, lakes or even stagnant ponds of freshwater, in the soil and even in some decaying matters. Many are solitary organisms, but some are colonial; some are free-living, others are sessile; and some species are even parasites of plants...