Blue Zoo: Planarian Flatworm

Featuring one amazing marine animal per week.

The flatworms, Phylum Platyhelminthes, are mostly very small, or even microscopic. This group also includes many parasites – such as the famously icky parasitic tapeworm - but they’re not all bad.

In the Class Turbellaria, you will find flatworms that are free-living – that is, they are not parasites dependent on a host. Sometimes they are called planarian flatworms; most of them live in the ocean. 

A variety of planarian flatworm belonging to the order Polycladida. Photo by Stephen Childs via Flickr, Creative Commons License. 

They are very simple animals. Their digestive system only has one opening – so the mouth is used for excreting waste as well as ingesting food. Since they don’t have a circulatory system, the gut has branches so that nutrients can be transported directly around the body.

They also lack a respiratory system. In order to breathe, they must be able to pass oxygen and carbon dioxide in and out of their bodies through their skin by diffusion. In order for this to work, even the largest flatworms are extremely thin. 

Polycladid flatworms are often brightly colored - a toxic warning to predators. Photo by Jens Petersen via Wikimedia Commons, Creative Commons License.

The order Polycladida includes some of the biggest species. “Big” for a flatworm usually means just several centimeters at most, but these are spectacular critters nonetheless. They swim and crawl gracefully by rippling their bodies (watch here), and often are very brightly colored. This is a display that tells potential predators that the worm is toxic.

Polyclads are often mistaken for nudibranchs due to their size and bright colors. However, nudibranchs are molluscs and not worms. Flatworms can be identified easily because, unlike nudibranchs, they do not have gills on their backs or other appendages.

Moving the sides of its body in a rippling motion, a flatworm crawls over corals. Photo by Silke Baron via Flickr, Creative Commons License. 

All planarian flatworms have very interesting reproductive habits. For one thing, they are able to regenerate lost body parts. In fact, a worm that is cut in half, will grow into two whole new worms. Some of them can also clone themselves without being cut – they just divide in two on their own! Scientists are studying turbellarian species to see if they can develop medical techniques to aid injured humans.

Flatworms aren’t just limited to asexual reproduction (reproducing alone). They can also reproduce sexually (with a mate). Almost all planarians are simultaneous hermaphrodites. This means that they have both male and female organs at the same time. When two flatworms mate, they both have a chance to be the father or the mother. Mating involves a process called penis fencing, in which both worms battle, trying to inseminate the other.

Learn more about penis fencing from the video below! 



Photo by Stephen Childs via Flickr, Creative Commons License. 

Photo by Stephen Childs via Flickr, Creative Commons License. 

Primary source:
Ruppert EE, Fox RS, Barnes RB. 2004. Invertebrate Zoology, A functional evolutionary approach, 7th ed. Brooks Cole/Thomson, Belmont, CA.

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