The phylum Platyhelminthes represents all flatworms in the kingdom Animalia. Flatworms are relatively primitive animals that represent an early lineage in the kingdom. Unlike the even more primitive phylum cnidaria, all flatworms of the phylum Platyhelminthes are bilaterally symmetrical, having an anterior, posterior, dorsal, and ventral end and two seemingly identical lateral surfaces. However, flatworms are the only type of bilaterally symmetrical animal that lacks any type of body cavity, a fluid filled space between the digestive tract and the body wall. In the flatworms, this area is filled with tissue. Flatworms have primitive brains, comprised of clusters of nervous tissue. Two long nerve cords run the length of the body, and they branch to form small nerves running throughout the body. Another hallmark of flatworms is a lack of a complete digestive tract. Flatworms take in food and excrete waste from one single hole. The opening in the gastrula never fully evolves in flatworms to make a tract that travels from one hole in the anterior end to another hole in the posterior end, which would make the tract complete. There are four classes that make up the phylum Platyhelminthes, each with unique structures and living styles.
Here are the classes in the phylum Platyhelminthes. Click on any of the names to see the characteristics of the class: