Phylum - Platyhelminthes (Flatworms)

Platyhelminthes is a phylum of relatively simple unsegmented, soft-bodied invertebrate animals. Flatworms do not have a body cavity called a coelum. They have an incomplete digestive system and no specialized circulatory or respiratory organs. They must use their flat shape to allow oxygen and nutirants to pass through their skin to all parts of their bodies. Flatworm are the first of the simple animals to show bilateral symmetry. Bilateral Symmetry will divide an organism into mirror image halves. There is an approximate reflection symmetry. Often the two halves can be referred to as the right and left halves.


Platyhelminthes are divided into four classes: Turbellaria, which are mostly non-parasitic animals such as planarians and other free living marine worms. Predators that live in water or in shaded, humid terrestrial environments such as leaf litter. The other three are entirely parasitic Cestoda (tapeworms) Adult cestodes live as parasites in the digestive systems of fish or land vertebrates, including humans. Trematoda (flukes) trematodes have complex life-cycles part of its life is lived inside a host animal. Monogenea are external parasites, infesting aquatic animals, and their larvae metamorph into the adult form upon finding a suitable host.

Photo Credit: These images are a work of the Centers for Disease Control and Prevention, part of the United States Department of Health and Human Services. As a work of the U.S. federal government, the image is in the public domain.

Diagram by:  Adrienne L