(i) Efficiency in transportation : All higher animals require efficient blood vascular system to supply the absorbed nutrients to different parts of body quickly. Blood transports oxygen and nutrients to the different parts of body as it contains haemoglobin, as compared to water vascular system which does not have it.

(ii) Immunity : However, blood provides immunity to body against unfavourable changes in the body because it has antibodies. But there is less immunity in animals of water vascular system. Thus, blood vascular system is more efficeint.