two chambers - fishes (atrium and ventricle)
three chambers - amphibians (two atria and ventricle)
also reptiles have three chambers with incomplete separation in ventricle
crocodiles, birds and mammals have four chambers - two atria and two ventricles (complete separation)
other differences: arthropods and many other groups of invertebrates have an open circulatory system which means that vessels are open and blood/other fluid circulates partially in body cavity
