Planarians lack dedicated respiratory and circulatory systems. this deficiency does not cause a problem because planarians lack dedicated respiratory and circulatory systems. this deficiency does not cause a problem because they lack mesoderm as embryos and, therefore, lack the adult tissues derived from mesoderm their body cavity, a pseudocoelom, carries out these functions their flame bulbs can carry out respiratory and circulatory functions none of their cells are far removed from the gastrovascular cavity or from the external environment