Planarians are free-living aquatic flatworms that are staples of high school biology labs.  The species I was able to photograph, Dugesia tigrina, is fairly small, growing up to about an inch in length when stretched out.

Planarians are utterly adorable.  Their heads have cute little eyespots (ocelli) that sense light and auricles (the triangular extensions) that reportedly sense water currents.  The eyespots lack lenses and a retina, so these cute little worms aren’t looking up at you and seeing your face, but they can detect the intensity and direction of light, allowing them to swim away from light (which is one of the easiest behaviors to observe in them; shine a light on them, and they’ll swim directly away from it). And when they move, they glide through the water with serpentine elegance.

The dark portions of the eye are not actually the photosensory nerves.  Instead, the dark portions are pigment-filled cells that partially surround the photosensory neurons, shading them from one side (thus allowing them to detect the direction of light without a lens, retina, or movable eye).

While many flatworms are parasitic, these planarians are not; they’re free-living omnivores that swim around in freshwater ponds nomming on whatever they can find predators feeding on small insects and other invertebrates they’re able to capture (see comment thread for citations).   In the lab we frequently feed them small pieces of liver or thymus.

The way flatworms feed is just awesome.  Instead of having a mouth at their head, they extend a tube (their pharynx) from the middle of their body and latch this tube onto their food.  They then “suck” the food up through this tube and into their digestive tract.

Speaking of guts, flatworms’ digestive tracts aren’t built like ours are: they have just a single opening that leads to and from their digestive track. This contrasts with our style of digestive tract, which has two openings: a mouth and an anus.  The planarian style of digestive tract is called a gastrovascular cavity, and it can be seen in the following image of a preserved planarian slide:

And yes, this does mean that digested food has only one way out: through the same opening that they used to get the food in.

Planarians are used in biology labs primarily thanks to their easy availability from biological supply houses ability to regrow tissues from traumatic injuries: when cut in half they can regrow the other half of their bodies. This is because while they can reproduce sexually using sperm and eggs, they can also reproduce asexually via fragmentation. Fragmentation is a reproduction mechanism wherein an organism literally pulls itself in half, with both halves growing into complete new organisms. This leads to the classic high school biology “experiment” wherein students cut flatworms in half and wait for them to regrow. We won’t be doing that here. But this picture of two flatworms swimming next to each other almost looks like it 🙂

I get live planarians each semester to show my biology classes, but sadly most students just give them a passing glance. Next time you get a chance to observe these cuties, put them in a dish of water, get a dissecting microscope and some liver, and plan to spend some time with them. They’re great fun!