Single-celled species are organisms that consist of only one cell, such as bacteria, archaea, and protists. These organisms carry out all the necessary functions for life within a single cell, including obtaining nutrients, reproducing, and responding to their environment. Single-celled species can be found in a wide range of habitats, from the depths of the ocean to the soil in our backyards. Despite their simple structure, single-celled species play crucial roles in ecosystems and have diverse and complex life cycles. **

Single-celled organisms do not have complex visual systems like multicellular organisms. They lack specialized structures such as eyes or light-sensitive cells. However, some single-celled organisms can detect changes in light intensity and direction, allowing them to move towards or away from light sources. This ability is known as phototaxis and is a simple form of light perception rather than true vision. **

No, fruit flies are not single-celled organisms. They are actually small insects belonging to the family Drosophilidae. Fruit flies have complex multicellular structures, including a nervous system, digestive system, and reproductive system. They are commonly used in scientific research due to their short life cycle and genetic similarity to humans. **

No, bacteria are not always single-celled organisms. While most bacteria are unicellular, there are some species of bacteria that can form multicellular structures or colonies. These multicellular forms of bacteria can work together to perform specific functions, such as forming biofilms for protection or sharing nutrients within the colony. However, the majority of bacteria are indeed single-celled organisms. **

Single-celled organisms known as Turritopsis dohrnii, also called the "immortal jellyfish," are believed to be biologically immortal. This species has the ability to revert back to its juvenile polyp stage after reaching maturity, essentially restarting its life cycle. This process can occur repeatedly, allowing the jellyfish to potentially live indefinitely under the right conditions. **

Single-celled organisms are considered immortal because they can continuously divide and reproduce without aging. Unlike multicellular organisms, they do not undergo the process of senescence, which is the gradual deterioration of cells and tissues over time. Additionally, single-celled organisms have the ability to repair and regenerate themselves, allowing them to potentially live indefinitely under favorable conditions. This immortality is a result of their simple structure and ability to replicate their genetic material with high fidelity. **

Single-celled organisms are still present because they have been able to adapt and thrive in a wide range of environments. Their simplicity allows them to reproduce quickly and efficiently, making them resilient to changing conditions. Additionally, single-celled organisms play important roles in various ecosystems, such as nutrient cycling and decomposition. Their ability to survive in extreme conditions, such as high temperatures or high salinity, also contributes to their continued existence on Earth. **

No, humans are not single-celled organisms. Humans are multicellular organisms, meaning they are made up of many different types of cells that work together to form tissues, organs, and organ systems. Single-celled organisms, on the other hand, are made up of just one cell that carries out all the functions necessary for life. **