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Protozoa (sg.: protozoan or protozoon; alternative plural: protozoans) are a polyphyletic group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic debris. [1] [2] Historically, protozoans were regarded as "one-celled animals".
Single-celled microorganisms were the first forms of life to develop on Earth, approximately 3.5 billion years ago. [ 30 ] [ 31 ] [ 32 ] Further evolution was slow, [ 33 ] and for about 3 billion years in the Precambrian eon , (much of the history of life on Earth ), all organisms were microorganisms.
The black rat is a reservoir host for bubonic plague.The rat fleas that infest the rats are vectors for the disease.. In biology and medicine, a host is a larger organism that harbours a smaller organism; [1] whether a parasitic, a mutualistic, or a commensalist guest ().
Life originated as marine single-celled prokaryotes (bacteria and archaea) and later evolved into more complex eukaryotes. Eukaryotes are the more developed life forms known as plants, animals, fungi and protists. Protists are the eukaryotes that cannot be classified as plants, fungi or animals. They are mostly single-celled and microscopic.
A unicellular organism, also known as a single-celled organism, is an organism that consists of a single cell, unlike a multicellular organism that consists of multiple cells. Organisms fall into two general categories: prokaryotic organisms and eukaryotic organisms. Most prokaryotes are unicellular and are classified into bacteria and archaea
At the base of the ocean food web are single-celled algae and other plant-like organisms known as phytoplankton. The second trophic level (primary consumers) is occupied by zooplankton which feed off the phytoplankton. Higher order consumers complete the web. There has been increasing recognition in recent years concerning marine microorganisms.
A single parasite species usually has an aggregated distribution across host animals, which means that most hosts carry few parasites, while a few hosts carry the vast majority of parasite individuals. This poses considerable problems for students of parasite ecology, as it renders parametric statistics as commonly used by biologists invalid.
For this reason, collective bacteria and other microorganisms in a host are often called "flora". [172] The term "bacteria" was traditionally applied to all microscopic, single-cell prokaryotes.