Why do simple animals, such as flatworms and sea stars, survive without special circulatory systems but more complex organisms, like reptiles and humans, cannot?

Flatworms get most of their oxygen through diffusion.

Explanation:

The flatworms and the tapeworms all are single-celled organisms and don't have any complicated structure or compound type of the cells and thus can survive with the presence of the gills like that of the complex organism have. Their flatness gives them a greater surface area to absorb more oxygen and they can take the oxygen from the skins.