Paramecia are single-celled organisms usually less than 0.25 mm (0.01 inch) in length, containing nuclei and different other organelles. They belong to the class Ciliata of the phylum Protozoa. Body is slipper-shaped, and is covered by distinct pellicle. The body is somewhat cylindrical. The anterior end is slender and blunt while the posterior end is thick and pointed. A paramecium digests vegetative matter moving through the water with the help of locomotive organs, called cilia. Studying the physical structures of paramecium is important because the species has been classified depending on the microscopic morphology instead of their sexual characteristic.
Paramecia have fine protoplasmic cilia covering the body and arranged in definite longitudinal rows for locomotion. Minuscule cilia are used in locomotion as well as during feeding. These hair-like filaments beat rhythmically to propel food particles into the mouth. They sometimes exhibit characteristic movement by changing its shape slowly by waves of contraction.
There is a definite digestive tract and a ciliated peristomial oral groove leading to the cell mouth and cell gullet, as well as a temporary anus. The oral groove is located on the ventral side and there is no chlorophyll; holozoic type of nutrition does occur.
The excretory apparatus is more complicated. There are two large contractile vacuoles occupying at fixed positions, one at either end of the body. Each vacuole is surrounded by six to eight radiating vacuoles. They are not only help in excretory functions but expel water out of the cell in order to maintain the salt concentration of a paramecium.
Beneath the flexible body membrane, there lies a thin and clear layer of ectoplasm which encloses the inner fluid portion of the cytoplasm. The ectoplasm contains granules, food vacuoles, and numerous spindle shaped trichocysts for defense or attachment to substratum. Trichocysts are released by electrical, chemical or mechanical means.
There are two nuclei of different sizes and functions. The smaller micronucleus is kidney-shaped which usually remains dormant and becomes active during sexual processes, while the large nucleus is round shaped and participates in the daily activities from growth to reproduction. Both types of nuclei contain genes that carry genetic and hereditary information of paramecium. Without micronucleus the organism cannot reproduce so micronucleus is essential for survival. The large nucleus also stores genes, but it is mainly the center of all metabolic activities of the cell.
The body surface is covered by tough pellicle and bears numerous openings for the trichocysts.