Taxis response is to move towards or away from the stimulus. If the organism moves towards the stimulus the taxis is positive, while if it moves away the taxis is negative. For example, flagellate protozoans of the genus "Euglena" move towards a light source. This reaction or behaviour is called "positive phototaxis", since phototaxis refers to a response to light and the organism is moving towards the stimulus. Many types of taxis have been identified, including: Depending on the type of sensory organs present, a taxis can be classified as a "klinotaxis", where an organism continuously samples the environment to determine
History of model organisms like Wendell Stanley wanted to perform. Before the use of these simple organisms molecular biologist had comparatively complex organisms to work with. Today these viruses, including bacteriophages, are used extensively in genetics. They are critical in helping researchers to produce DNA within bacteria. The Tobacco Mosaic Virus has DNA that stacks itself in a distinctive way that was influential in Watson and Cricks development of their model of the helical structure for DNA. Both the community of insects and the viruses were a good start to the history of model organisms, but there are yet still more players involved. At