The Unicellular (one celled) Animals
Details of animals which consist of only one cell. Preview of life of animals seen with microscope.
There's no doubt about it that the earliest forms of life were simple unicellular organisms and in that respect protozoans must be considered the earliest animals. Many of these have survived the longest geological periods with very little change. In the simplest form, protozoans have bodies composed of a single mass of cytoplasm or unit rarely reaching more than 1 mm across, containing all the structures found in a typical cell such as nucleus, mitochondria, etc. But the cell may attain complexity and may have intricate parts because, after all, this cell is the whole animal and it has to meet all the demands of life, such as, nutrition, respiration, locomotion, homeostasis reproduction and other such functions.
Amoeba is a typical example of a simple unicellular animal. Paramecium is a much more complicated protozoan, but still its body never attains the complexity found in larger animals. Protozoans are mostly aquatic, fresh water or marine, but some are found in damp places, and a number of them are parasites. Inspite of their different kinds of habitats, they rarely show any structural adaptations in a way larger animals do. Thus, it is difficult to make inferences about their mode of life merely from their structure.
Except for the parasitic protozoans which don’t have any specific means of locomotion, most of them can move by different structures, such as pseudopodia, flagella, or cilia etc. many protozoa are able to tide over unfavorable conditions by making resistant structures called cysts. Regeneration is also common in them because of these two properties; they have a remarkable capacity for survival. They reproduce asexually by binary fission, multiple fission of budding, sexually by means of isogamy, an-isogamy or conjugation.
There are about 45000 species of protozoans. Some of these are shelled, as for example Difflugia and only these kinds of protozoans can be found in fossil form. While most protozoa live solitary lives, some get together to form colonies. Other examples include plasmodium (malarial parasite), Trypanosoma which causes sleeping sickness and Vorticella, a colonial sessile ciliate etc.