The simplest animals that are bilaterally symmetrical and triploblastic (composed of three fundamental cell layers) are the Platyhelminthes, the flatworms. Flatworms have no body cavity other than the gut (and the smallest free-living forms may even lack that!) and lack an anus; the same pharyngeal opening both takes in food and expels waste.

Platyhelminthes are triploblastic, bilaterally symmetrical, dorsoventra, acoelomate flatworms with organ grade of construction without a definite anus, circulatory, skeletal or respiratory system but with Protonephridial excretory system and mesenchyme filling the space between the various organs of the body.

Characteristics of Platyhelminthes Platyhelminthes have the following important characteristics: They are triploblastic, acoelomate, and bilaterally symmetrical. They may be free-living or parasites. The body has a soft covering with or without cilia. Their body is dorsoventrally flattened without any segments and appears like a leaf.

Platyhelminthes are commonly known as flatworms or tapeworms. They are a group of soft-bodied invertebrate animals. As a matter of fact, there are around 20,000 species of these animals. A few of these live as parasites on humans and other animals.