The flatworm Planaria and the roundworm Caenorhabditis elegans are both lab organisms that are called worms. However, they are not closely related. Flatworms (phylum Platyhelminthes) and roundworms (phylum Nematoda) are both called worms, but there are many differences between these two types of organisms.
A flatworm has a thin, dorsoventrally flattened body. Roundworms are more cylindrical in shape and tapered to a fine point at one end.
Roundworms have a rigid outer covering called a cuticle that they must repeatedly shed as they grow. Flatworms do not have a cuticle and often have cilia on the surface of their bodies.
Flatworms are acoelomate, which means they do not have a body cavity. Roundworms are psuedocoelomate, which means they have a body cavity between their mesoderm and endoderm layers.
The flatworm has a gastrovascular cavity, with only a single opening that functions as both mouth and anus. The roundworm has a complete digestive tract, with two separate openings for the mouth and anus.
Roundworms have longitudinal muscles (oriented lengthwise down the worm) that they contract to bend their bodies in a thrashing motion. The gliding locomotion of a flatworm is powered by many tiny cilia on the outer surface of its body.
While there are plenty of free-living flatworms and roundworms, there are parasitic forms of both flatworms and roundworms that cause disease in humans. Blood flukes are flatworms that cause schistosomiasis, which ranks second only to malaria in terms of morbidity and mortality, worldwide. Other disease-causing flatworm parasites include the lung flukes and liver flukes. Roundworms that cause disease include Ascaris, a large intestinal worm that can grow to the size of a pencil, as well as hookworms and whipworms.