Classes Trematoda and Monogenea represent the parasitic flukes. These flatworms have similar structures to class Turbellaria but their mouth is at the anterior end. These worms have suckers in the mouth that attach to blood vessels near the intestine. The smaller female fluke fits into a groove on the male, and they are often found attached, a position in which they can copulate freely. The eggs pass out in the host's feces and the larva can infect another organism and assexually divide to reproduce. Often, a fluke life cycle will involve living in more than one host. Many flukes can infect humans, and cause diseases such as schistosomiasis, infecting millions in developing countries.
These classes belong 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 classes Trematoda and Monogenea are two of four classes in this phylum.