The three main characteristics used for a hierarchical classification are:
(i) Complexity of cell structure, i.e., type of cell-prokaryotic or eukaryotic. As a eukaryotic cell has membrane-bound organelles including a nucleus, the cellular processes can be carried out efficiently in isolation from other cells. On the other hand, the organisms without a clearly demarcated nucleus and other organelles need to have very different biochemical pathways. This would naturally have a great effect on every aspect of cell design. Moreover, the nucleated cells would have the capacity to participate in making a multicellular organism as they are capable of taking up specialised functions.
(ii) Body organisation, i.e., whether the organism is unicellular or multicellular. In a eukaryotic multicellular organism, cells that group together to form a single organism use the principle of division of labour. In this type of body design, all cells would not be identical. Rather, groups of cells will carry out specialised functions. Thus, this makes a very basic distinction in the body designs of organisms. As a result, an amoeba will be very different in its body design from a fish.
(iii) Mode of nutrition: autotrophic or heterotrophic. Plants make their own food while animals depend on plants or other animals for their food. For this they will definitely have different body design.
