Which organisms are called primitive and how are they different from the so-called advanced organism?

Organisms labeled as "primitive" are generally those that have retained many of the characteristics of early ancestral forms throughout evolution and have not undergone complex changes. These organisms are often simple in structure and function. Examples include bacteria, amoeba, and certain algae. They are typically unicellular, though some multicellular organisms can also be considered primitive if they have simple structures.

On the other hand, "advanced" organisms have undergone significant evolutionary changes and have developed more complex structures and functions. These include multicellular animals and plants with specialized tissues and organs, higher brain functions, and more sophisticated mechanisms for interacting with their environment. Humans, mammals, birds, and flowering plants are examples of advanced organisms.

The differences between primitive and advanced organisms include:

Complexity: Advanced organisms have more complex body structures and functions, with specialized cells and tissues, while primitive organisms are simpler with fewer specialized structures.
Cellular Organization: Most advanced organisms are multicellular with highly organized cells and tissues, while primitive organisms are often unicellular or have simple multicellular structures.
Reproduction: Advanced organisms often have complex reproductive systems, while primitive organisms reproduce in simpler ways, often asexually.
Metabolism and Adaptation: Advanced organisms typically have more complex metabolic pathways and are better adapted to a wide range of environments, whereas primitive organisms often survive in specific niches.
Genetic Diversity: Advanced organisms generally have more genetic material and can exhibit more complex genetic behaviors, such as sexual reproduction, leading to greater diversity.
It's important to note that calling an organism "primitive" does not imply it is inferior; it simply has a lifestyle and structure that has worked well for it over millions of years, and it has not needed to change dramatically. The terms "primitive" and "advanced" are somewhat outdated in scientific discourse because they can be misleading. Evolution doesn't work towards a goal of "advancement"; rather, it's about organisms adapting to their environments in various ways. So, while these terms are useful for general understanding, modern biology tends to focus on the specifics of each organism's adaptations and evolutionary history.