where does gas exchange take place in the gills
Fish gill Gills usually consist of thin filaments of tissue, branches, or slender tufted processes that have a highly folded surface to increase surface area. The high surface area is crucial to the gas exchange of aquatic organisms as water contains only a small fraction of the dissolved oxygen that air does. A cubic meter of air contains about 250 grams of oxygen at STP. The concentration of oxygen in water is lower than air and it diffuses more slowly. In a litre of freshwater the oxygen content is 8Â cm3 per litre compared to 210 in the same volume of air.[8] Water is 777 times more dense than air and is 100 times more viscous.[8] Oxygen has a diffusion rate in air 10,000 times greater than in water.[8] The use of sac-like lungs to remove oxygen from water would not be efficient enough to sustain life.[8] Rather than using lungs "Gaseous exchange takes place across the surface of highly vascularised gills over which a one-way current of water is kept flowing by a specialised pumping mechanism. The density of the water prevents the gills from collapsing and lying on top of each other, which is what happens when a fish is taken out of water."[8]
Extracellular digestion Joint intracellular and extracellular digestion In hydra and other cnidarians, the food is caught by the tentacles and ingested through the mouth into the single large digestive cavity, the gastrovascular cavity. Enzymes are secreted from the cells bordering this cavity and poured on the food for extracellular digestion. Small particles of the partially digested food are engulfed into the vacuoles of the digestive cells for intracellular digestion. Any undigested and un-absorbed food is finally thrown out of the mouth.[8]