1) Respiration in non-chordate animals occurs through diffusion or specialized respiratory organs depending on the species. Single-celled organisms rely on diffusion, while sponges also use diffusion and water circulation.
2) Invertebrates like worms, insects, and arthropods use structures like trachea, book lungs, and gills for respiration. Trachea are a branching tube system that deliver oxygen to insect tissues. Book lungs are found in some spiders and scorpions.
3) Molluscs generally use gills called ctenidia. Echinoderms rely on diffusion through tube feet extensions, while some develop dermal branchiae. Respiratory pigments like hemoglobin