The vertebrates have developed not only internal  skeletons, but other features as well which suit them for active life. Their respiratory system, whether it takes the form of gills or lungs, is beautifully adapted to the active interchange of oxygen between the external medium and a blood, and the latter is made much more efficient than the average invertebrate blood by having its oxygen-carrying respiratory pigment concentrated in corpuscles. This blood is pumped through a closed system of vessels, rather than through an open system of irregular sinuses, by a heart of relatively high efficiency. (Wiener 1989 :53-54)
There is another limitation on the insect, which is due to its method of respiration and circulation. The heart of the insect is a very poor and weak tubular structure, which opens, not into well-defined blood vessels, but into vague cavities or sinuses conveying the blood to the tissues. This blood is without pigmented corpuscles, and carries the blood-pigments in solution. This mode of transferring oxygen seems to be definitely inferior to the corpuscular method. (Wiener 1989 :55)
WIENER, Norbert. 1989 . The human use of human beings: cybernetics and society. London: Free Association Books.