What are the advantages of the appearance of the circulatory system? Why is it closed in annelids,

What are the advantages of the appearance of the circulatory system? Why is it closed in annelids, but not in mollusks and arthropods?

The circulatory system appeared in animals due to the complication of the organism. The size of the body has increased and it has become much more difficult for cells to receive the right amount of oxygen. In simple animals that do not have a circulatory system, oxygen enters the cells through diffusion. That is, the molecules pass through the membrane walls of the cell and enter the desired organelles. Ringworms are ways of surviving body damage. If they had an open circulatory system, then damage to the body would lead to death.