Try it on paper first. A hint, the key idea and the worked solution are here when you need them.
Name the breathing organ of each, and say whether it takes oxygen from air or from water: (a) a rohu fish, (b) a grasshopper, (c) a frog on land, (d) an earthworm. What do all these surfaces have in common?
Gases are exchanged by diffusion across a large, thin, moist surface. Water holds far less dissolved oxygen than air, so fish move a lot of water over their gills. Insects use tracheae, land vertebrates use lungs, and plants use stomata and lenticels. By day photosynthesis uses more carbon dioxide than respiration makes; at night only respiration goes on.
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
(a) The rohu fish uses its gills, with the oxygen dissolved in water.
(b) The grasshopper uses tracheae, air tubes that open at spiracles. It takes oxygen from air.
(c) The frog uses its lungs and its moist skin. It takes oxygen from air.
(d) The earthworm uses its moist skin, with oxygen from the air in the soil.
Answer: gills (water), tracheae (air), lungs and skin (air), skin (air). Every one of these surfaces is large for the animal, thin and moist.
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