The correct option is A.
Explanation
Gas exchange mechanisms in the animal kingdom vary significantly based on the habitat and the level of organization of the organism. These mechanisms are specialized structures designed to facilitate the diffusion of oxygen and carbon dioxide between the animal and its environment.
- Statement 1 is Incorrect: Insects primarily utilize a tracheal system for respiration. This system consists of a network of tubes (tracheae) that transport atmospheric air directly to the tissues. Gills are respiratory organs specialized for aquatic environments and are not the standard respiratory mechanism for terrestrial insects.
- Statement 2 is Correct: In the context of biological classification, "vascularised bags" refers to lungs. While most aquatic arthropods use gills, certain groups within the broader classification of arthropods (specifically terrestrial ones or specific semi-aquatic transitions) and other animal groups rely on vascularised bags (lungs) for atmospheric gas exchange. In the context of the provided options and biological accuracy regarding diverse respiratory structures, the presence of vascularised structures is a recognized mechanism for gas exchange in various animal phyla.
- Statement 3 is Correct: Gills are the predominant respiratory organs for aquatic arthropods (such as crustaceans) and molluscs. These structures are highly efficient at extracting dissolved oxygen from water, which is essential for survival in aquatic habitats.
Key Takeaway: Respiratory structures are habitat-dependent: insects use tracheal tubes, aquatic arthropods primarily use gills, and terrestrial forms utilize vascularised bags (lungs).