Try it on paper first. A hint, the key idea and the worked solution are here when you need them.
How many gill openings would you see on one side of the head of a lamprey with pairs of gill slits, a dogfish with pairs, and a katla? What is the total for the three?
Cyclostomata: no jaws, 6 to 15 pairs of gill slits, no scales, no paired fins. Chondrichthyes: cartilage, ventral mouth, separate gill slits with no operculum, placoid scales, no air bladder. Osteichthyes: bone, terminal mouth, four pairs of gills under an operculum, air bladder. Amphibia: moist skin without scales, cloaca, a three-chambered heart, gills, lungs and skin for breathing.
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.
Step 1. The lamprey. Seven pairs means seven slits on each side, all open: .
Step 2. The dogfish. No operculum, so five separate slits on each side: .
Step 3. The katla. Its four gills on each side lie under one operculum, which has one opening: .
Step 4. The total. .
Answer: lamprey , dogfish , katla ; in all on one side.
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