Members of the phylum, Platyhelminthes are called Flatworms. Within this group are classes of worms. In class Turbellaria there are the Planarian worms which are free-living. In classes Trematoda and Crestoda are the Flukes and Tapeworms which are identified as parasitic worms. The differences between a free-living worm and a parasitic worm are abundant. Free-living worms have organ systems for digestion, excretion, response and reproduction. Whereas, parasitic worms have evolved from free-living ancestors. As these worms evolved into parasites they either modified or lost these important features. As a result. parasitic species are typically simpler in structure. Free-living flatworms can be carnivorous creatures that feed on small aquatic animals or they can be scavengers that feed on recently dead animals. Just like the cnidarians, flatworms have a digestive cavity with a single opening or mouth which food or waste passes. Near the mouth is a muscular tube called a pharynx which extends out of the mouth to pump food into the digestive cavity or cut. Diffusion will then transport the digested food to other parts of the body. Parasitic worms feed on blood, tissue fluids or pieces of cells within a host's body. These creatures feed on already digested food from the host. Therefore, many parasitic species of worms have a less complex digestive tract as they don't need to break down or hunt food, as they just absorb it from the host they are invading. Certain species will contain a pharynx which is used to pump food throughout the body where others like tapeworms have no digestive tract at all. Most flatworms have more complex structures for detecting and responding to external stimuli than those of cnidarians or sponges. For example, in free-living flatworms, a head encloses several ganglia or a group of nerve cells that control its nervous system. Free-living worms also have two long nerve cords that run from the ganglia down the side of the body. Eyespots are or groups of cells that can detect changes in amount of light are located on the anterior end of the body. The nervous system in free-living flatworms allow them to gather information from their environment, as they can use this information to locate food and find dark hiding places throughout their environment. Whereas, parasitic worms interact a lot less with their external environment, resulting in a less complex nervous system. The reproduction cycles of free-living worms consist of being hermaphroditic meaning having both the male and female reproductive organs. Free-living worms can also split into two identical daughter worms through fission which is a way of asexual reproduction. In contrast, parasitic worms have an alternation of generations based life cycle. They start by reproducing sexually in the blood vessels of a humans intestines, embryos are released and passed out with the feces. If they get into water, the embryos develop into swimming larvae and infect an immediate host (snail) After asexual reproduction new larvae are released from the snail into water and they will infect humans, the primary host, by burrowing through their skin