Question

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

Answer 1

Hint:Cladograms (is a diagram which is used to represent the hypothetical relationship between groups of elements) are an important source to hypothesize and classify the origin and the future of an organism. This helps the scientists to design the evolutionary scenarios to find out the primitive trait.

Complete answer
To answer this question, we have to know about the primitive organisms. Primitive organisms are the organisms that have ancient body designs and do not change much during evolution. They have simple body structure and bodily functions. Single cell organisms and the eukaryotic organisms are advanced forms of primitive organisms.
Now, the primitive organisms are different from advanced organisms based on their body structure, the advanced organisms have complex body structure and function compared to that of primitive organisms. A primitive trait is the original trait of the organism with common ancestor while the advanced trait observed some notable change. It is not about sophistication or superiority, it depends on the adaptiveness of the organisms. The primitive defines the character that appeared first in the organisms that have the common ancestors of a clade and it moves largely to the recent organisms of the clade while advance means the character that undergoes evolution within the subgroup of the clade.

Note:The vertebrates belong to the clade that have advanced legs, and advanced traits although it is a feature that is found in the clade. While in tetrapods clade they appear to have primitive legs as they are inherited from the same ancestor.