Final answer:
The star-shaped organelle near the posterior end of the Euglena is called a stigma or eyespot, used for sensing light.
Step-by-step explanation:
The star-shaped organelle near the posterior end of a euglena is called a stigma or eyespot. This organelle plays a pivotal role in sensing light, enabling the euglena to move in response to light sources, a behavior known as phototaxis. The organism uses its two flagella to propel itself in the water, and it's the eyespot that guides this movement. In addition to the eyespot, euglena possess many unique features such as a pellicle that provides rigidity and flexibility, as well as chloroplasts for photosynthesis in those species that are photoautotrophic.