Try it on paper first. A hint, the key idea and the worked solution are here when you need them.
Which classes of vertebrates have a heart with two chambers, three chambers and four chambers? Why does a warm-blooded animal need a heart with four chambers?
Pisces: scales, gills, 2 chambers. Amphibia: moist skin, 3 chambers. Reptilia: dry scales, 3 chambers (4 in the crocodile). Aves: feathers, 4 chambers, warm-blooded. Mammalia: hair, milk, 4 chambers, warm-blooded.
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. Two chambers. Pisces.
Step 2. Three chambers. Amphibia and Reptilia (the crocodile has four).
Step 3. Four chambers. Aves and Mammalia.
Step 4. Why four. Four chambers keep blood rich in oxygen apart from blood poor in oxygen. A warm body uses a lot of energy, so it needs a full supply of oxygen.
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