Flatworms are the simplest of the worm groups, unsegmented soft-bodied invertebrates. There are about 20,000 species in this group. They occupy a range of habitats consisting of marine, freshwater, and even damp terrestrial environments.  They can be free living or parasitic. A parasite lives off of another living thing called a host and can be harmful. One of the best known flatworms is the tapeworm. The tapeworm can get into a person's digestive tract and grow to enormous lengths. The tapeworm then eats off the host and is dangerous to the host as it grows and consumes more of the host and its food. (http://www.virted.org/Animals/Flatworm.html)

This is an example of a flatworm, but is not what was observed in this experiment. Meiofauna flatworms are microscopic, and were identified threw their movement.