Platyhelminthes do not have a circulatory system.  There are no veins, lymph nodes, or arteries. Beacuse they are fairly small in size and are so flat, they are able to breath through skin, which is just an integumemt. This breathing alloes animal to exchange oxygen ehich is outside bringing it into the animal and used it wherever needed.

There is no need of circulatory system since they do not have lungs.