-Nematodes are basically known otherwise as simple roundworms. In the phylum Nematoda there is no circulatory system. The species within this phylum have there own different way of circulation. Since they don’t have a circulatory system, their fluids are dispersed into the pseudocoelom. Some species from the phylum Nematoda are nematodes, roundworms, and threadworms. Roundworms don’t have any heart or vessels in their body.