Question

The correct hierarchical order with respect to the level of organisation in animals is
a) Organ system, organ, tissue, cell
b) Cell, tissue, organ, organ system
c) Tissue, cell, organ, organ system
d) Cell, organ, organ system, tissue

Answer 1

Hierarchical order explains the evolution of animals from simplest to complex structures. Complex animals can perform many independent functions. The level of organisation begins with the basic unit of life.

Complete answer:
The Cells are the structural and functional unit of life. Initially the organisms were classified as Unicellular and Multicellular. Unicellular organisms have only one cell for all its functions while multicellular organisms have many cells for its functions. Thus multicellular organisms are much more evolved than unicellular.
The main problem in Multicellular organisms was in spite of many cells, there was no division of labour. This means the cells are involved in many functions. Thus the need of proper grouping of cells was important.

Thus tissues were formed.( tissue is derived from word tissue meaning woven).Thus tissues are defined as groups of cells together.

As the organisms became more complex, the tissues were organised in groups known as organs, which were responsible for performing different physiological functions. The organs then formed organ systems and thus complex organisms were formed. This is the hierarchy seen in the evolution of animals.

Thus the correct ans is Cells→Tissues →Organs →Organ system i.e. option B.

Note: The hierarchy is also a major characteristic to study the different phyla of the animal kingdom.
This is seen in the tabular form below:

Different phyla of Animal kingdom	Cellular level
Porifera	Tissue level
Coelenterates	Tissue level
Ctenophora	Organ
Platyhelminthes	Organ system
Nemathelminthes	Organ system
Annelids	Organ system
Arthropods	Organ system
Molluscs	Organ system
Echinodermata	Organ system
Hemichordates	Organ system
Chordates	Organ system