Scientists study both the flatworm Planaria and the roundworm Caenorhabditis elegans in laboratories, using them as test subjects, and while they may seem similar, they have some distinct internal and external differences. Flatworms (phylum Platyhelminthes) and roundworms (phylum Nematoda) differ in shape, means of locomotion, how their digestive systems function, and in the ways they are harmful to human beings. While both species are called worms, they are not closely related.