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
Skeletal changes of organisms transitioning from water to land were already completed in many aquatic organisms. As aquatic tetrapods began their transition to land, several skeletal changes are thought to have occurred to allow for movement and respiration on land. Some adaptations required to adjust to non-aquatic life include the movement and use of alternating limbs, the use of pelvic appendages as sturdy propulsors, and the use of a solid surface at the organism’s base to generate propulsive force required for walking. The Osteolepiformes and Elpistostegalia are two crown groups of rhipidistians with respect to the tetrapods. The development of skull roof and cheekbone patterns in these organisms match