a photographic atlas for the Bass Strait region
Photographic Atlas

About flatworms

The flatworms - Phylum Platyhelminthes - comprise a very diverse group of worms, with over 10,000 species described. Included within this large phylum are parasitic flatworms, including tapeworms and liver flukes. However most marine flatworms seen by divers belong to the Polycladida or "polyclads". Polyclads are all marine and free-living (in other words, they are not parasites). Unlike many other kinds of worms, such as earthworms, flatworms do not have a body cavity containing organs - instead the entire body is a solid mass of tissue. There is no circulatory system either, and flatworms rely on diffusion to convey nutrients and waste products between cells. No doubt that is why flatworms are so flat and thin - no part of the interior of the body is far from the surface.

Cycloporus australis [photo: John Chuk]Complete description and identification of a flatworm is a complex process, because the internal anatomy (arrangement of reproductive and other organs) differs considerably between different types of flatworm. These fundamental internal differences are important for flatworm classification, but can only be deduced by making a series of very thin serial sections of specially preserved speciemens. These cross-sections reveal narrow traces of internal structures, and many of them in combination can be used to draw a diagram of the otherwise murky interior of a flatworm. Think of taking a series of very thin slices through your garage to try and find the lawnmower and you get an idea of how difficult this can be, and of the advantages of colour photography for flatworm taxonomy. Ultimately there will be no avoiding dissections and sectioning of specimens by experts, but we hope in many cases to learn to tell one species from another simply from photographs.

The flatworm illustrated here, and tentatively identfied as Cycloporus australis [photo: John Chuk], shows the distictive colour patterns seen in many flatworms.

The world of a flatworm must be a violent existence. They are voracious predators which attack and digest prey with eversible mouthparts called a pharynx. Oysters are a favourite food, but no doubt there are many prey preferences and other associations with invertebrates that are yet to be discovered. Many flatworms are strikingly coloured, are toxic to other invertebrates, or mimic other invertebrates. Or all three. Sex in flatworms is bizarre. Flatworms are hermaphrodites (each worm has both male and female reproductive systems) and sex may involve "penis fencing" whereby each worm tries to spear sperm into the other. Sort of like a "pin the tail on the donkey" game, albeit with significant family planning consequences.

All these details, and much more besides, are elaborated in a fascinating and beautifully illustrated book recently written by Leslie Newman and Lester Cannon: Marine Flatworms - the world of polyclads (see resources and contacts pages for details). However, much of what we know has been gleaned from observing the flatworm fauna of tropical waters. In this atlas project we hope to begin to make similar discoveries in our own Bass Strait region.