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|Chapter Seven: The Classification of Unicellular Organisms|
Sporozoans are parasites; they live at the expense of another organism. One of their significant characteristics is that they lack cilia and flagella. They generally obtain nutrients by absorbing organic molecules from the host organism.
Sporozoans often have very complicated life cycles. Take, for example, the Plasmodium vivax, the organisms responsible for the disease malaria. It grows inside of a mosquito's stomach, and, after reaching maturity, migrates to the mosquito's salivary glands where it releases thousands of small cells called sporozoites. When the mosquito bites a person, the sporozoites are transmitted into the human bloodstream. They enter the liver where they divide and enter red blood cells. Eventually, they break free from the red blood cells as gametes (causing the fever associated with malaria). If a mosquito bites the person at this time, the gametes can be ingested into the mosquito's stomach where they unite and grow into new organisms.
As their name suggests, must members of the phylum Ciliophora (they are called ciliates) have great amounts of cilia. Ciliates may also have structures called trichocysts, organelles which can be discharged from the cell. Trichocysts may be used to anchor the organism or to capture prey by paralyzing it with a trichocyst with a poisoned tip.
Ciliates, which can live in both freshwater and marine environments, usually consume bacteria or other protists. They also feed on organic material which may be floating in the surrounding water.
An interesting aspect of ciliates is their possession of two nuclei. One is called the micronucleus, and the other is known as the macronucleus. It is currently believed that the micronucleus is involved with sexual reproduction, while the macronucleus controls such aspects of the organism's life as growth, respiration, and asexual reproduction.
The most well known ciliate is the paramecium. The body of a paramecia is completely covered by cilia, allowing for efficient locomotion. They have a gullet (also known as an oral groove) through which large food particles may pass. Smaller particles are consumed through the cell membrane into vacuoles by phagocytosis. Most paramecia also have a contractile vacuole.
Paramecia may reproduce asexually through mitotic division, but they may also engage in a sharing of genetic information with another paramecium through a process called conjugation. In this process, the two paramecia align their gullets and allow material in the nucleus to pass through. In this manner, they keep some of their initial DNA but obtain new DNA, so that new paramecia formed through mitosis later on may have slightly varying characteristics.