Paramecium  is well-known ciliate protozoans. It demonstrates  a very high degree of cellular differentiation, exhibiting many complex organelles which have been designed to perform specific functions for the organism. Not only its structure show specialization, but its reproductive activity is also complex. There are about 10 species of Paramecium. Its common species are P.caudatum and P.aurelia.

1.     Habit and Habitat:

                    It’s a free living organism having cosmopolitan (world-wide) distribution. It lives in stagnant water of freshwater, ponds, pools, ditches, lakes and slow flowing rich in decaying organic matter

                               Paramecium cadatum is a unicellular and microscopic protozoan. It measures 170 to 290 µm up to 300-350 µm. It is visible to the naked eye. Its body has a constant elongated, slipper-like shape, so it’s also called slipper animalcule. Anterior part of the body is blunt and broad and posterior end is thick, pointed and cone shaped, widest part is just below the middle. Body is asymmetrical showing well defined oral or ventral surface and convex dorsal or aboral surface.

                            The whole body is covered by thin, firm, flexible membrane called pellicle. Pellicle is made up of gelatinous substance. It gives shape of the animal but is elastic to permit contraction.

                           Body is covered by numerous, small hair-like projections called cilia, arranged in longitudinal rows. The length of cilia is uniform throughout the body ( a condition called holotrichous), but there are a few longer cilia at the posterior end of body. These form caudal tuft of cilia (hence the name caudatum). Cilia have the same structure as flagella, an outer protoplasmic sheath or plasma membrane with nine double longitudinal fibrils, in a peripheral ring. Two central fibrils, which are thinner than the outer fibrils. Each cilium arrives from a basal granule. At the base the cilia has a diameter of 0.2 µm (2000 A ̊).

3. Cytostome:

                         On the ventro-lateral side is a large oblique, shallow depression called oral groove or peristome which gives the animal asymmetrical appearance. The oral groove lends into a short conical funnel shaped depression called vestibule. Vestibule leads to an oval shaped opening called cytostome. It is followed by a long opening  called cytopharynx; which leads to the oesophagous  - that leads to food vacuole.

                       The cytopyge or cytoproct lies on the ventral surface, almost vertically behind the cytostome. Undigested food particles are eliminated through the cytopyge.

                       The cytoplasm is differentiated into a narrow peripheral layer of clear and dense plasmegel, called ectoplasm and an inner large central mass of granular and semifluid plasmasol or endoplasm.

                      It forms a firm, clear, thin and dense outer layer. It contains the trichocysts, cilia and fibrillar structures and is bound externally by a covering called pellicle.

                      Small spindle shape bags are embedded in the ectoplasm filled with a refractive, dense fluid having swelling substance. At the outer end there is a conical head on spike. Trichocysts lie perpendicular to ectoplasm.

                    Endoplasm is the voluminous part of the cytoplasm, contains many granules as well as other structures and inclusions such as mitochondria, vacuoles etc. other students includes nuclei, contractile vacuole, food vacuole etc.

                       It is a ellipsoidal or kidney like shaped nucleus. Densely packed with chromatin granules (DNA). It is a vegetative nucleus, controls vegetative functions.

                     Small compact and spherical,  found close to macro nucleus. Fine chromatin granules and threads uniformly distributed throughout the structure. It controls reproduction. Number varies with species to species. Nucleolus is absent in caudatum.

           There are two fluid filled contractile vacuoles, one each at the ends of the body close to dorsal surface. Their position is fixed between the ectoplasm and endoplasm. They are temporary organs disappearing periodically. Five to twelve radical canals are connected  to each contractile vacuole. Each radial canal consists of a terminal part, a long ampula and a short injector canal which opens into the contractile vacuole. The canal communicate with large part of the body and takes up liquids and pour into the vacuole. Thus the vacuole increase in size and the liquid discharged outside during systole through the permanent opening (pore) in the pelged. The two vacuoles contract irregulargly, the posterior C.V contract more rapidly because it is close to the cytopharynx and more water comes to it. The main function of contracted vacuole is osmoregulation and probably involves respiration and excretion.