Question

Sol. [Key Concept: Sponges exchange gases by simple diffusion]

Mechanisms of breathing vary among different groups of animals depending mainly on their habitats and levels of organisation.

Lower invertebrates like sponges, coelenterates, flatworms, etc., exchange $O_{2}$ with ${CO}_{2}$ by simple diffusion over their entire body surfaces. Earthworms use their moist cuticle and insects have a network of tubes (tracheal tubes) to transport atmospheric air within the body.

Gills are used by most of the aquatic arthropods and molluscs.