How Paramecia Get Food
A striking feature of the Paramecium is the oral groove along one side of the cell. The cell also has a definite front end, or anterior part, which is rounded. The rear end, or posterior part, is more pointed. The oral groove runs from the anterior toward the posterior end.
Cilia line the oral groove and cause the Paramecium to spin around its long axis as it swims. More important, these cilia sweep food particles back toward the mouth pore. This pore, in turn, opens into a funnel like gullet, which reaches into the cytoplasm. Bacteria and other foods forced into the gullet are formed into a food vacuole.
When a food vacuole reaches a certain size, it breaks away from the gullet. The movement of the cytoplasm carries the vacuole around the cell. During this movement, foods are digested and absorbed, as in amebas. Undigested particles are passed out through the anal pore. This tiny opening in the pellicle is found near the posterior end. It is completely closed except when in use.
How Paramecia Remove Wastes
A contractile vacuole for removing extra water is found near each end of the cell. Around each vacuole are canals leading into the cytoplasm. These canals enlarge as they fill with water, which is then passed on to the central part of the vacuole. From there, the water is emptied through an opening in the cell surface. Biologists estimate that in just 30 minutes a Paramecium pumps out a volume of water equal to the whole cell content. Most other waste products appear to diffuse out the cell membrane.
Response in Paramecia
Like amebas, paramecia have no specialized sense organs. Yet paramecia, too, respond to conditions around them. Except when feeding, the cells swim constantly. When they bump into something, they reverse, turn, and swim off in a new direction. This kind of trial-and-error response is called the avoiding reaction. They also avoid areas of extreme heat or cold, areas that lack dissolved oxygen, or areas that contain foreign chemicals. They also tend to stay in areas that have a good food supply.
Paramecia also have a defense response. Just inside the pellicle are the trichocysts, which appear as tiny lines when seen through a microscope. When a larger protozoan approaches, the trichocysts shoot special threads of protoplasm out into the water. These threads are quite long and give the Paramecium a bristly appearance. This response may be caused by chemicals given off by the larger organism.
Reproduction in Paramecia
Paramecia have two different kinds of nuclei, both found near the center of the cell. A large macronucleus controls normal cell activity. A much smaller micronucleus functions during reproduction. Some species have more than one micronucleus.
Paramecia reproduce by cell fission. The macronucleus and micronucleus divide during this process. Under good conditions, fission may occur two or three times a day.
From time to time, paramecia also reproduce by a type of sexual reproduction called conjugation. After a series of nuclear divisions and other steps, two paramecia exchange micronuclei, then continue to divide by fission. This causes mixing of genetic material and provides more variation. Variation increases the chances for the species to survive.