The blood of small worms and mollusks has no oxygen-binding substance. Later animals had molecules capable of transporting large amounts of oxygen. In many invertebrates these molecules are dissolved in the plasma. Hemocyanin, a copper-containing protein chemically unlike hemoglobin, is found in some crabs and other lower animals. Hemocyanin is blue in color when bound to oxygen and colorless without it.
Some invertebrates have hemoglobin in solution in the plasma.
The blood is pumped round the body by the heart. When amphibians emerged from the water onto the land, the way the blood circulated had to change. It no longer flowed through the gills, but now had to flow through the lungs, which were in a different part of the body. In reptiles the blood coming from the lungs, which is rich in oxygen, is mixed up in the heart with blood coming from the body, which is low in oxygen. This blood is then pumped back to the body and the lungs.