Cestodes, or tapeworms, are a class of worms characterized by their flat, segmented bodies. The segments are called proglottides and hold both male and female reproductive organs, allowing self-fertilization. Proglottides that contain fertilized eggs break off or dissolve, passing the eggs out of the host. Adult tapeworms typically reside in the intestinal tract of vertebrates, attaching themselves to the mucosal lining with hooks or suckers on their scolex, or head. They do not possess a digestive tract, or alimentary canal, of their own, but absorb nutrients through their tegument, or skin, from partially digested food as it passes through the host.
Common tapeworms that frequent humans are Taenia saginata, Taenia solium, and Diphllobothrium latum. These parasites use intermediate hosts—cattle, swine, and fish respectively. Many parasites infect an intermediate host organism while in a developmental form, but they do not grow to maturity until they have been transmitted to the final or definitive host. In the Taenia species, the eggs are passed into cattle or swine through infected soil. They develop into an intermediary stage, called a cysticercus, that embeds in the muscle and connective tissue of the animal. Infected animals that are processed for meat but improperly cooked still harbor the parasite and pass the cysticerci on when consumed by humans. The cysticeri develop into adult tapeworms that attach to the intestinal lining of the host. The cysticerci of T. solium can, themselves, cause medical complications. Instead of developing immediately into adult tapeworms, these cysticerci can migrate to any organ of the body, commonly ending up in the muscles or brain. A serious infection in the brain can lead to mental complications, including seizures and personality changes.