|Many marine Turbelarian flatworms are quite beautiful|
The phylum is broken down into a number of different classes, all of which are quite different.
This class is mostly made up of free-living species, though a small number are parasitic or commensal (species that live off of another but do not cause the other species any harm). Though the vast majority of turbellarians live in water, there are a few species that are able to live on land. My favorite are these terrestrial land planarians.
|Pork tapeworm proglottid, full of eggs|
This class includes the tapeworms. Tapeworms can be quite nasty parasites, living in the gut of mostly vertebrate species. Nutrition is obtained via the outer body covering. They have a "head" or scolex that is little more than an attachment point, covered in hooks and suckers. Following that is a number of segments known as proglottids, whose main purpose is reproduction. It is estimates that up to one hundred thirty-five million people are currently living with tapeworm infections. Pork tapeworms can be especially nasty as worms can form cysts in the brain if a human becomes infected.
A relatively under-studied class, most species contained within are parasites of fish gills. Previously placed both in with the cestodes and the trematodes (which will be discussed next), molecular studies have placed them in their own separate class.
|This fluke causes schistosomiasis|
Overall, the greatest concern surrounding Platyhelminthes revolves around the parasitic species, especially those which infect humans. Both cestodes and trematodes can be very serious and some are difficult to treat. Schistosomiasis is especially concerning due to its high infection rate. Research is being done to control the spread of the disease. The infections that are seen are usually restricted to underdeveloped countries, with few if any cases popping up in countries like the United States.
Source is Biology of the Invertebrates. Images are from Wikimedia Commons and are under Creative Commons licenses or are copyright free: one, two, three.