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This is a model of the large, complex ciliate called Paramecium. These unicellular organisms are often found in water containing bacteria and decaying organic matter. Note the large, kidney-shaped macronucleus that controls most of the metabolic functions of the organism. Located close to (and often within a depression on the macronucleus) is the much smaller micronucleus, which is involved in reproduction. As in other freshwater protozoans, contractile vacuoles are used to remove excess water that enters the organism by osmosis. In addition to these organelles, note the ciliated oral groove that directs food to a permanent opening called the cytostome ("cell mouth"). Some species also maintain a permanent opening to outside called a cytoproct ("cell anus"). Embedded within the pellicle (a rigid outer covering that provides support for the cilia which project through it), are numerous thread-like structures called trichocysts. Upon mechanical or chemical stimulation, these trichocysts can be discharged (as shown on the model) to produce long, sticky protein threads that remain attached to the organism. It is believed that these structures can be used for defense. |
