Although paramecia don't have skin the way that a multicellular animal often does, a membrane called the pellicle holds them together. The pellicle is covered by many small, hair-like extrusions known as cilia. The cilia move in waves to propel paramecia through their watery homes. The outside of the paramecium is also the site of the oral groove, which sweeps the bacteria and other microorganisms paramecia feed on into the cell mouth to be digested.
The inside of a paramecium is filled with a liquid called cytoplasm. Within the cytoplasm is the paramecium's gullet, into which food particles pass after entering through the cell mouth. Food collected in the gullet is eventually transformed into a food vacuole, which floats out of the gullet into the cytoplasm, where enzymes digest it to provide energy for the paramecium's life functions. Also present inside the paramecium are the macronucleus and one or more micronuclei, both of which contain genetic material.
Paramecia can reproduce in one of two ways: asexually or through a process called conjugation. In asexual reproduction, the macronucleus divides and the paramecium splits into two individual organisms that are genetically identical. When conjugation occurs, two paramecia temporarily fuse together and exchange genetic material from their micronuclei. The two paramecia separate and new macronuclei form from the mingled genetic material. Conjugation is typically followed by asexual reproduction, leading to the possibility of four daughter paramecia.
Trichocysts are elongated, rod-like structures that scientists have observed being released from paramecium cells in response to certain electrical, mechanical or chemical stimuli. Some scientists think the trichocysts serve as tiny weapons used to secure the paramecium's prey microorganisms. Others theorize that, while trichocysts serve that purpose in other genera, the trichocysts of the paramecium are used to anchor the organism to a certain position to facilitate the feeding process.