 The lymphatic system, on the other hand, is an open system providing an accessory route for excess interstitial fluid to be returned to the blood.The more primitive, diploblastic animal phyla lack circulatory systems.The blood vascular system first appeared probably in an ancestor of the triploblasts over 600 million years ago, overcoming the time-distance constraints of diffusion, while endothelium evolved in an ancestral vertebrate some 540–510 million years ago.