The Playhelminthes is a flatworm with a flattened, bilaterally symmetrical body, meaning that if you cut the Platyhelmthes in half, it would be a mirror image. Along with its bilateral body, the Platyhelmthes has an anterior, posterior, dorsal, and ventral end. The Platyhelmthes is divided into three different classes including Turbellaria, Trematoda, and Cestoda. This phylum does not have a skeletal, circulatory, and respiratory system but does have an excretory system where the flatworm uses "flame cells" and its associated ducts to regulate its excretory system. There are 20,000 known species of Platyhelmthes, most of which are parasitic, and are different than most billateral symetrical animals due to it's unique digestive system.
PLATYHELMINTHES DIGESTIVE SYSTEM
This Phylum, as mentioned before, has a different and unique kind of digestive system compared to other flatworms. This is because it only has one opening for it's digestive system which is also used for it's mouth but not anus making their digestive tract incomplete. The Platyhelmthes uses it's mouth, pharynx, and intestine for its digestive system. It works but sucking food into the intestine where phagocytic cells of the gastrodermis complete digestion. The Cestoda, one of the three classes of the Platyhelmthes, does not have a digestive system and the rest of the parasitic classes rely on the host for digestion. Since the animals cannot feed, digest, and eliminate undigested residues simultaneously, food cannot be processed continually.
Class Turbellaria is free-living, aquatic, carnivorous, and has an incomplete digestive tract. Their digestive system is intracellular, meaning that their digestive system occurs inside a cell. Their digestive system starts in the mouth where it then leads to the pharynx and then into a temporary space that take in food particles by phagocytosis.
The Trematoda is also known as a fluke and is an endoparasite meaning that they live in their host's body. Their digestive system is similar to the Turbellarian digestive system but they don't have the Turbellarian's extensible pharynx. However, because the Trematoda lives in digestive system of their hosts, many Trematodes lack a digestive system because they can simply absorb pre-digested matter from their hosts.
The Cestoda is a tapeworm with a long and flat body made up of segments called proglottids. It is parasitic and an endoparasite. Cestodes have evolved to have no digestive system and rely on their hosts gut to absorb different nutrients. For this to happen, microscopic wrinkles cover the Cestoda to make more room for nutrients to be absorbed.