Sponges belong to the Phylum Porifera. They are the simplest multicellular animals characterized by a cellular level of organization and a water transport system (canal system).
Due to the lack of specialized respiratory organs like gills, lungs, or tracheal systems, sponges rely on the direct exchange of gases with the surrounding water.
The exchange of oxygen ($O_{2}$) and carbon dioxide ($CO_{2}$) occurs across the thin cell membranes of the body surface via simple diffusion, driven by the concentration gradient maintained by the constant flow of water through the body.
Final Answer: simple diffusion over their entire body surfaces
AI generated content. Review strictly for academic accuracy.