. Principles of modern biology. Biology. Responsiveness in Single Cells - 195 MICRONEEDLE. STIMULUS. Fig. 11-4. Response of an amoeba, pricked by a microneedle. The specimen moves away in a co- ordinated manner, indicating that the excitation is conducted from the site of stimulation to all parts of the cell. not act in a concerted fashion as the amoeba alters the direction of its locomotion. In the amoeba, apparently, any part of the cell may serve temporarily as receptor, conductor, or effector, and scarcely any definite differentia- tion of these parts can be recognized. In the amoeba, howe
. Principles of modern biology. Biology. Responsiveness in Single Cells - 195 MICRONEEDLE. STIMULUS. Fig. 11-4. Response of an amoeba, pricked by a microneedle. The specimen moves away in a co- ordinated manner, indicating that the excitation is conducted from the site of stimulation to all parts of the cell. not act in a concerted fashion as the amoeba alters the direction of its locomotion. In the amoeba, apparently, any part of the cell may serve temporarily as receptor, conductor, or effector, and scarcely any definite differentia- tion of these parts can be recognized. In the amoeba, however, it has recently been shown that the tip of an advancing pseudopodium is consistently negative in relation to other parts of the cell surface, which indicates that the action potential has its origin here. Perhaps the commonest specialized recep- tors in unicellular organisms are the red- colored chromatophores, or "eyespots," that are found in many green flagellates (Figs. 9-7 and 10-4). Each chromatophore is a small pigmented granule, which is generally re- garded as a photoreceptor. Probably many of the light-seeking responses that are char- acteristic of the phytoflagellates are initiated in these receptor structures. The cilia and flagella of the Protozoa and free-swimming algae serve as effectors of mechanical response, and probably also as receptors of tactile stimuli, since frequently these parts are first to make contact with ex- ternal objects. Many protozoans, especially ciliates, also possess internally placed con- tractile fibrils, called myonemes (Figs. 11-5 and 11-6). These intracellular effectors act more or less like the muscle fibers of higher animals. By shortening quickly and forcibly, the myonemes enable a one-celled organism. RETREATING to change its shape, sometimes in very drastic fashion. Specialized conductors are relatively rare among unicellular organisms. The undiffer- entiated protoplasm of these cells propagates an excitation at a co
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