A diverse group of animal-like protists (eukaryotic organism that consists of a single cell). Well known examples are amoeba, paramecium, and euglena. Protozoa were once considered to belong to the animal kingdom (Animalia) alongside the metazoans; however, this is no longer the case. Today, protozoans, along with ciliates, mastigophorans, and apicomplexans, are classed as animal-like protists. Biologists have identified about 65,000 species of protozoa, almost half which are extinct species from fossils.
Sensory capabilitiesMany species of protozoa have physiological mechanisms for monitoring conditions in their environment. Free-living varieties often have a localized region of pigment called an eyespot, which can detect changes in both the quantity and quality of light. Certain protozoan species also sense physical and chemical changes or obstacles in their environment.
Cyst formationMany protozoans can survive in harsh conditions or severe changes in the environment, such as nutrient deficiency, drought, decreased oxygen concentration, or pH or temperature changes, by forming cysts. A cyst is a dormant form characterized by a hardened external covering inside which metabolic activity has ceased. When favorable environmental conditions return, a protozoan emerges from the cyst state and resumes metabolic activity.
Related category• ZOOLOGY
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