Planarian
Planarian is the common name given to many non-parasitic flatworms of Turbellaria class. Planaria are common to many parts of the world, living in both saltwater and freshwater. Some species are terrestrial and are found under logs, in soil, and sometimes on plants in humid regions. These animals move by beating cilia on the ventral dermis, allowing them to glide along on a strand of mucus. The planarian has very simple organ systems. The digestive system consists of a mouth, pharynx, and a structure called a gastrovascular cavity. The mouth is located in the center of the underside of the body. Digestive enzymes are secreted from the mouth to begin external digestion. The pharynx connects the mouth to the gastrovascular cavity. This structure branches throughout the body allowing nutrients from food to reach all extremities. Planaria eat living or dead small animals that they suck with their muscular mouths. Food passes from the mouth through the pharynx into the intestines where it is digested, and its nutrients then diffuse to the rest of the body.
Tapeworms
Tapeworms belong to the platyhelminths phylum known as the cestodes, which are entirely parasitic.They consist of an anchoring organ called scolex which attaches them to the intestinal wall with suckers to help it hold strong. Adult tapeworms do not feed off humans, but rather take in our digested food. Tapeworms lack a digestive tract and absorb nutrients directly across the skin or cuticle. Health problems occur when the tapeworm becomes too large and blocks the bowel, robbing them off of nutrients, thus preventing them from staying alive.Tapeworms may grow to a reasonable size and, because they are found deep in the tissue, may start taking up valuable space. If the cyst settles in the brain, eye or liver, normal function of these organs may be severely restricted. So, if the cyst affects the digestion of nutrients in tapeworms, it won't be long for the tapeworm to die from starvation.
Fluke
The fluke, a parasitic flatworm of the trematoda class within the phylum Platyhelminthes, are similar to tapeworms. Instead of the cilia, external sense organs, and epidermis of the free-living flatworms, adult flukes have sucking disks with which they cling to their hosts and an external cuticle that resists digestion by the host. In most species, the adult flukes absorb nutrients through the digestive system. In some worms, their digestive system is reduced or completely absent, so food is absorbed through the cuticle. Adult flukes are commonly hermaphroditic, which means that each individual possesses both male and female reproductive organs. However, they reproduce sexually.