Organisms who belong to the phyla Platyhelminthes are also called flat worms. Flat worms are unsegmented bilaterally symmetrical worms. The majority of flatworms are parasitic and have three germ layers. Flat worms have a nervous system that consists of the head that is attached to nerve cords that spread across the body. Flatworms lack a respiratory or circulatory system as they absorb what they need through their body wall. Excretion is handled by "flame cells" in the protonephridia. The protonophridia is absent in many organisms, but it takes place of the worms anus. Flat worms can reproduce sexually or asexually, but the majority of flat worms have developed ways to avoid self-fertilization.
Myers, P. 2002. "Platyhelminthes" (On-line), Animal Diversity Web. Accessed February 28, 2011 http://animaldiversity.ummz.umich.edu/site/accounts/information/Platyhelminthes.html.
