I have been keeping a land planarian, Bipalium adventitium, since about the end of September. In part 3 and part 4 of this series, I chronicled my attempts to feed slugs to it. In fact, I now have enough data for a short paper that I am co-authoring with Megan P.
According to several published studies (citations below), the preferred, and perhaps the sole, prey of this planarian are earthworms. Bipalium adventitium is a passive-aggressive predator. It doesn't seem to chase after its prey, but attack one only if it happens to bump into one while foraging.
Several studies have also described the predatory behavior of B. adventitium (citations below). When a hungry planarian makes contact with an earthworm, it climbs on the worm and everts its pharynx. The earthworm is quickly immobilized apparently by the planarian’s pharynx. The planarian then starts to consume the worm. Here is a photo showing the planarian eating a worm. Note that the planarian's mouth is not in or anywhere near its head; it's on the ventral side near the middle of the body.
The light colored tissue covering a portion of the earthworm’s body is the planarian’s everted pharynx (arrow).
During this process the worm continues to move its head or tail for up to 45 minutes, but is otherwise incapable of freeing itself from the planarian.
And here is a short film* of an earthworm being devoured alive by B. adventitium. The planarian, light brown, is on top of the darker colored worm. You can see one end of the worm moving. During the 2nd half of the clip, the planarian's head is also visible.
I don't know if earthworms can feel pain; if they do, this must be a horrendous way to die. But, as Richard Dawkins said in his book River out of Eden, "nature is not cruel, only pitilessly indifferent" to such petty affairs. Planarians must eat too.
Part 6 is here.
*Once again, I apologize for the poor quality of the movie. This is the best I can do with my present camera.
Dindal, D.L. 1970. Feeding behavior of a terrestrial turbellarian, Bipalium adventitium. American Midland Naturalist 83:635-637.
Ducey, P.K. & Noce, S. 1998. Successful invasion of New York state by the terrestrial planarian, Bipalium adventitium. Northeastern Naturalist 5:199-206.
Ducey, P.K., Messere, M., Lapoint, K. & Noce, S. 1999. Lumbricid prey and potential herpetofaunal predators of the invading terrestrial flatworm Bipalium adventitium (Turbellaria: Tricladida: Terricola). American Midland Naturalist 141: 305-314.
Ducey, P.K., McCormick, M. & Davidson, E. 2007. Natural history observations on Bipalium cf. vagum Jones and Sterrer (Platyhelminthes: Tricladida), a terrestrial broadhead planarian new to North America. Southeastern Naturalist 6: 449–460.
Fiore, C., Tull, J.L., Zehner, S. & Ducey, P.K. 2004. Tracking and predation on earthworms by the invasive terrestrial planarian Bipalium adventitium (Tricladida, Platyhelminthes). Behavioural Processes 67: 327-334.
Zaborski, E.R. 2002. Observations on feeding behavior by the terrestrial flatworm Bipalium adventitium (Platyhelminthes: Tricladida: Terricola) from Illinois. American Midland Naturalist 148: 401-408.