Flatworm
Animals called worms are not necessarily closely related, because the term "worm" refers to a type of adaptive body form that occurs in several different groups. Flatworms (phylum Platyhelminthes) and roundworms (phylum Nematoda) are both called worms, but there are many differences between these two types of organisms.
Shape
A flatworm has a thin, dorsoventrally flattened body. Roundworms are more cylindrical in shape and tapered to a fine point at one end.
Body Surface
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.
Body Cavity
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.
Digestive system
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.
Locomotion
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.
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