Gas Exchange
Hydras and Planarians
Small animals with large surface area
Most of their cells exchange gases directly with environment
Larger aquatic animals
Often have gills
-Finely divided vascularized outgrowths of inner body surface
-Gills of bony fishes
-Outward exllensions of pharynx
-Ventilation is brought about by combined action of the mount and gill covers (operculum)
Countercurrent Exchange