The forth class of flatworms, class Cestoda, include all tapeworms. These flatworms have suckers and teeth that grasp the host in the small head. Behind a short neck are repeated parts of the worm, each containing reproductive structures that contain both eggs and sperm, which can pass out through the host's feces, like fluke eggs. These repeated parts are in order, with the oldest parts farthest away from the head of the tapeworm. The pieces give the worm a ribbonlike structure, beneficial for absorbing nutrients from the intestine, where these parasitic tapeworms live.

This class belongs in the phylum Platyhelminthes, consisting of all flatworms. They are more evolved than the Cnidaria because they have bilateral symmetry, with a distinct anterior, posterior, dorsal, and ventral end, with a defined head. However, flatworms lack a body cavity, a fluid filled region between the epidermis and the digestive tract. Flatworms also lack a complete digestive tract, but instead only have one opening for substances to both enter and leave the body. This is because the gastrula opening in the development of a flatworm never fully evolves into a hole in the anterior and posterior end. The class Cestoidea is one of four classes in this phylum.