When we talk about microscopic animals, we usually mean 'unicellular' animals, a life form of minuscule size which consist of a single cell containing lots of tiny specialized organs known to biologists as 'organelles'... These organelles, analogous to the internal organs of higher animals, are responsible for the vital functions of the cell. Many people often call these things microbes, microorganisms or microscopic organisms...
If one looks closely at the many species of single-celled organisms, collectively known as Protista, one sees that they can be divided into one of the two categories, Protozoa (primitive animals) or Protophyta (primitive plants), according to their mode of nutrition. Protophytes synthesize the substances necessary for their existence by means of assimilatory pigments which act in the same way that chlorophyll does in plants, but Protozoons capture and digest their food (tiny organic particles) in the same way that animals do. Several species of Protista are capable of feeding themselves by both ways - which is why they cannot properly be classified as protozoons or protophytes.
The function of these microscopic animals are perfected to an incredible degree. Whereas in the higher animal cells are differentiated to form various types of tissue and tend to be less rather than more complex because of their specialization - the single cell of a protozoon has to be completely self-sufficient. It has to perform all the function necessary for survival - it has to feed itself, it has to remove organic waste, respond in various ways to environmental stimuli, possibly manufacture a protective shell or some form of ciliary apparatus for moving about and, of course, reproduce.
Protozoons occur absolutely everywhere. Their smallness equips them for colonizing every sort of environment, including the bodies of every single animal, which gives one some idea of just how important a role they play in ecology. There are various other animals, such as rotifers, which come into the microscopic class. Rotifers abound in pond water, soil and damp moss. Many of them are only a few hundredths of a millimeter long - the same order of magnitude as most protozoons. Studying rotifers under the microscope is a fascinating experience; they come in every possible shape, as weird and fantastic as anything in science fiction. The tardigrades is another strange group of microscopic animals.
In March of 2013, researchers reported data that suggested microbial life forms thrive in the Mariana Trench, the deepest spot on the Earth. Other researchers reported related studies that microbes thrive inside rocks up to 1900 feet (580 meters) below the sea floor under 8500 feet (2590 meters) of ocean off the coast of the northwestern United States. According to one of the researchers, "You can find microbes everywhere - they're extremely adaptable to conditions, and survive wherever they are.
---End of Post "Microscopic Animals"