Important Internal structures of Paramecium are given below:
1. Cytoplasm:
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Cytoplasm is bounded by pellicle and differentiated into outer dense zone, ectoplasm and inner semi-fluid zone, endoplasm.
Ectoplasm contains infraciliary system and the trichocysts, while endoplasm contains remaining cellular structures like mitochondria, golgi body, ribosomes, crystals, food granules, nuclei, contractile and food apparatus.
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2. Infracilliary system:
Pellicular alveoli are followed by infraciliary system consisting of basal bodies and kinetodesomata.
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Basal bodies:
The basal ends of each cilium are called basal bodies or kinetosome. The wall of basal body consists of a triplet of sub-fibers.
The basal bodies are self-duplicating units and progenitors of new cilia. They are either a centriole or its derivative.
Kinetodesmata:
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From the basal bodies a system of specialized striated fibrils is framed which is called kinetodesma.
Each kinetodesma contains 5 fibrils and all the kinetodesma together make a network of fibrils called kinetodesmata.
3. Trichocysts:
Trichocysts are peculiar rod-like or oval organelles present throughout the ectoplasm alternating with basal bodies and oriented at right angles to the body surface.
They consist of an elongated shaft and a terminal pointed tip called the spike or barb, covered by a cap. Shaft contains a fibrous protein trichinin.
Trichocysts are believed to help in discharging and anchoring the animal to a firm substratum when it feeds upon bacteria. Some zoologists believe that these are organelles of defence.
4. Nucleus:
In P. caudatum, there are two nucleus, a large macronucleus and a small micronucleus, the nuclear dimorphic condition.
The macronucleus is roughly kidney-shaped and bounded by a conspicuous nuclear membrane. It is polyploid and possesses many nucleoli and much more chromatin material (DNA).
It is derived from micronucleus but acts as somatic or vegetative nucleus and controls the day-to-day metabolic activities of the cell.
Micronucleus is small, diploid with a nuclear membrane and lodged in a depression on the surface of the macronucleus. It controls the reproduction of the organism.
5. Contractile apparatus:
It contains two contractile vacuoles, have somewhat fixed position in endoplasm. One vacuole lies near each end of body, close to the dorsal surface.
Each of them is surrounded by a circle of 6 to 10 long, narrow, spindle shaped radial canals extending far into cytoplasm.
Each contractile vacuole opens outside through a permanent pore in pellicle of dorsal side of body.
6. Food vacuoles:
Numerous food vacuoles are seen to move with streaming of endoplasm (cyclosis). They serve for intracellular digestion and distribution of food to the entire body.
7. Oral apparatus:
Oral groove leads ventrally and posteriorly as tubular vestibule and directly opens into buccal cavity.
In its turn it opens into a narrow gullet or cytopharynx through a narrow aperture, cytostome. The cytopharynx, at its proximal end forms a food vacuole.
Further, buccal cavity is bordered by a row of cilia forming an endoral membrane at right side. On the left side are three groups of long cilia viz., ventral peniculus, dorsal peniculus and quadrulus. All these structures collectively are called oral apparatus.
8. Cytopyge:
Near posterior end of body, a small portion of ectoplasm and pellicle is somewhat weak. Here at the time of egestion, a minute aperture called cell anus, cytopyge or cytoproct is visible.