Nematoda
Animals in the Nematoda phylum, such as nematodes, roundworms, and threadworms, lack a circulatory system. The species within the Nematoda phylum have their own way of circulation. Since these animals do not have a circulatory system, their fluids are dispersed into the pseudocoelom, a body cavity. Roundworms don’t have any heart or vessels in their body.