Various protozoa produce an exoskeleton in the form of shells, which are made up of siliceous, calcareous, or proteinaceous material. A characteristic type of covering called cyst occurs commonly in various protozoa.
The cyst protects the protozoan cell from environmental factors, such as high light intensity, high temperature, desication, food exudation, and anaerobiosis.
The cyst is also involved in the transmission of protozoan diseases. Like other eukaryotic cells, the cytoplasm of a protozoan cell is more or less homogenous substance and the various structures
(endoplasmic reticulum, Golgi bodies, mitochondria, ribosomes, blepharoplasts, food vacuoles, contractile vacuoles, and nucleus) are embedded within it.
Protozoa possess a more or less continuous network or canals and lacunae that give rise to endoplasmic
reticulum.
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