Question

what is mean of organisum

Answer 1

An organism may be defined as an assembly of molecules functioning as a more or less stable whole that exhibits the properties of life.

Answer 2

Primitive organisms are the one who appeared early on earth and have simpler structures and body plans than the advanced organisms. 

Advanced organisms are complex and have acquired new features morphologically and anatomically by evolution from simpler organisms to better adapt the changing environmental conditions. However its not always true that advanced organisms have complex structures. For ex. Mammals have fewer teeth than primitive fishes.