Flatworms are more complex than sponges and coelenterates. Flatworms and the rest of the animal phyla are organized into three distinct layers of tissues. Tissues from these layers form the organs of a flatworm. Flatworms are the simplest worms. They have flat bodies with definite right and left sides, a head, a back end, and top and bottom sides. All parasites, including parasitic flatworms, are believed to have origin as free-living forms and to have lost certain tissues and organs, as secondary effect of their parasitic existence, while developing adaptations of advantage to the parasitic way of life.
Some flatworms live in the freshwater while others live in seawater. However, most flatworms, such as the fluke and tapeworm, are parasites. They live in or on other organisms that supply their food. Free-living flatworm does not live in or on other organisms for its food. It uses its digestive system to break down food and has sense organs in its head to help it find food. The two eyespots on the planarianís head are sensitive to light. The bumps on the side of its head are sensitive to touch.
Flatworms make up the phylum Platyhelminthes. The phylum name is derived from two Greek words: platy, which means flat, and helminthes, which means worm. The 13,000 species of flatworms belong to three major classes. Members of the class Turbellaria are called free living, because they can live independently. Members of the class Trematoda, the flukes, and the class Cestoda, the tapeworms, are parasites. They derive nutrition from living hosts.
The most common flatworm is the planarian, which belongs to the class of Turbellaria. Planarians live in both aquatic and terrestrial environments. Some kinds grow to be 30 cm lon, and many are brightly colored.
Planarians have a spade-shaped head and a body covered with cilia. The planarianís mouth is in the middle of its body on the bottom side. The mouth, a tube-like pharynx and a large digestive cavity are parts of its digestive system. The planarian has a good way to eat food that is too large to fit in its mouth. It simply extends its pharynx through its mouth opening. The pharynx then draws the food inside the body to be digested in the digestive track. The planarian does not have an opening for removing wastes. Therefore, wastes must also pass through the mouth. Planarians eat dead animals or slow-moving animal organisms, including smaller planarians. Planarians move by laying down on the trail or slime from mucus-producing cells and using their cilia to propel themselves along the trail. A planarian will move away from light and toward food. These responses are made possible by the planarianís simple nervous system.
Planarians are hermaphrodites Ė that is, each animal has both female and male reproductive structures. Sexual reproduction occurs by a mutual exchange of sperm. In most animals females have ovaries, which are organs that make eggs. Males have testes which make sperm. Planarians have both ovaries and testes in the same individual. Usually the sperm do not fertilize eggs from the same worm. Instead, two worms mate and exchange sperm cells. After fertilization, the eggs pass out of the body to develop into new planarians. Asexual reproduction occurs if you cut a planarian - each half would regenerate a new half.
Tapeworms and Flukes
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 donít 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 donít 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 sheepís body in its feces, or solid wastes.