What are we referring to when we say 'simply organisms' or 'complex organism'?

Explanation

The terms .simple organisms. and .complex organisms. are referred to when we talk about evolutionary history of an organism. A simple organism is considered to be primitive if it has simple structural and functional organization. On the other hand, a complex organism under reference is considered to be more evolved form having complex structural and functional organization. These complex organisms are believed to have arisen from simpler forms in the course of evolution.