Phylum Plathelminthes includes also the tapeworms and flukes, parasites that can cause serious and sometimes fatal diseases among the higher animals. Members of both of these parasitic classes have a tough outer layer of the body that is resistant to digestive fluids and, usually, suckers or hooks on the anterior ends by which they fasten to their victims. The flukes feed through the oral sucker, but the tapeworms, which have no mouth but have digestive cavities, merely hang on and absorb food molecules. Tapeworms are found in intestines of many vertebrates, including man, and may grow as long as 15 to 20 feet. They cause illness not only by encroaching on the supply but also by producing wastes and by obstructing the intestinal tract. The most common human tapeworm, the beef tapeworm, infects people who eat undercooked flesh of cattle that have eaten fodder contaminated by these parasites.
A human life cycle includes all stages of life from the birth of parents to the birth of their children. The cycle is also known as a generation. Many parasites have complex life cycles. This parasitic invertebrate is a flatworm that infects both humans and pigs. The life cycle begins when people eat infected and improperly cooked pork. The tapeworms develop, reach adulthood, and lay eggs in the human intestine. The fertilized eggs are then released with human wastes. The fertilized eggs develop into larvae inside the pig. Larvae are an immature stage of tapeworms. If a human eats the infected pork the cycle is repeated. Human diseases can be controlled. Parasitic invertebrates have caused much human suffering and death around the world. Understanding the life cycles of parasites has helped to control the diseases that they cause.
You can be sure you never get a pork tapeworm by making certain that you don’t eat undercooked pork. Other ways to control pork tapeworms include disposing of human sewage carefully, inspecting meat, feeding hogs properly and treating people who already have tapeworms. Some parasitic invertebrate enters body when it burrows through the skin of the human foot. The life cycle can be broken by wearing shoes and by making sure human wastes don’t get in the soil.
Flukes belong to the class Trematoda. Flukes are parasites and are a serious health hazard in many areas of the world. Many kinds of flukes cause serious and even fatal diseases. Many flukes for years have a single host. Mature flukes have no cilia. Instead their skin is covered with a thick protective coating called cuticle, which prevents the host from digesting them. Flukes have a complex life cycle often involving two or more hosts. For example, the liver fluke spends the most of its life in the digestive tract of a sheep or other vertebrate. There are fluke lays of eggs, many of which are carried out of a sheep’s body in its feces, or solid wastes.