http://en.wikipedia.org/wiki/Red_blood_cell
The sequestration of oxygen-carrying proteins inside specialized cells (as opposed to oxygen carriers being dissolved in body fluid) was an important step in the evolution of vertebrates as it allows for less viscous blood, higher concentrations of oxygen, and better diffusion of oxygen from the blood to the tissues. The size of erythrocytes varies widely among vertebrate species; erythrocyte width is on average about 25% larger than capillary diameter, and it has been hypothesized that this improves the oxygen transfer from erythrocytes to tissues.
The only known vertebrates without erythrocytes are the crocodile icefishes (family Channichthyidae); they live in very oxygen-rich cold water and transport oxygen freely dissolved in their blood.While they do not use hemoglobin anymore, remnants of hemoglobin genes can be found in their genome.
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