Phylum Platyhelminthes

Many marine Turbelarian flatworms are quite beautiful

This group consists of what are called the flatworms. Indeed, the phylum name is from the Greek for flatworm. The group is rather varied and includes a number of parasites. Though many can be quite nasty parasites, there are also a number of species that are quite harmless and some that are even rather beautiful. There really isn't a single characteristic that can be used to define the phylum, except possibly the flat body. All flatworms are triploblastic acoelomates that are bilaterlally symmetrical. That is, they have three tissue layers but no body cavity and can only be evenly divided into mirror-image halves in one place. This is the first animal phyla that I have discussed that is triploblastic. It is also the first protostome species. This is a division among the animals based on very early development.

The phylum is broken down into a number of different classes, all of which are quite different.

Class Turbellaria
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

Class Cestoda
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.

Class Monogenea
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.

Class Trematoda

This fluke causes schistosomiasis

The best known members of this class are commonly known as flukes. One example, the blood fluke, causes schistosomiasis, which has an estimated two hundred million sufferers and kills approximately eight hundred thousand people each year. Trematodes often have quite complex life cycles, requiring up to three hosts and multiple changes in body form for the full cycle. Snails are an extremely common part of these life cycles, usually serving as an intermediate host before the parasite passes on to another host.  Common names of fluke species usually imply where the fluke lives, such as the blood flukes and liver flukes that are seen in humans. The group I have been discussing are known as Digeneans due to the two intermediate hosts seen in their life cycles. Trematodes also include a small group known as Aspidogastreans. These eighty species are parasites on molluscs and share traits with both monogeneans and digeneans.

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.