Structural organisation in animals attains different levels as cell-organ-organ system. What is missing in this chain? Mention the significance of such an organisation.

(N/A) The missing level in the chain is 'tissue'. The hierarchy is: $Cell \rightarrow Tissue \rightarrow Organ \rightarrow Organ System \rightarrow Organism$.
Significance:
$1$. Division of Labour: Different tissues and organs are specialized to perform specific functions, which increases the efficiency of the organism.
$2$. Complexity: It allows organisms to perform complex physiological processes (like digestion, respiration, and circulation) that are impossible for single-celled organisms.
$3$. Coordination: The organisation allows for better coordination and regulation of body activities through nervous and endocrine systems.