Types of Gas Exchange Systems in Animals

(A) Unicellular organisms exchange gases with the surrounding water directly across the cell membrane.
(B) Some multicellular aquatic animals use the body surface as an exchange surface; the blood (red) transports gases to and from the surface.
(C) Many multicellular aquatic animals have specialized evaginated gas exchange structures (gills).
(D) A few aquatic animals, such as the sea cucumber, use invaginated exchange areas.
(E) Most true air breathers have lungs, specialized invaginated areas that depend on a blood transport system.
(F) Most land arthropods have tracheal systems, invaginated tubes that carry air directly to the tissues without the intervention of a blood transport system.