A platyhelminth is a flatworm, from the Greek roots πλατύ- (platy) meaning "flat" and ἑλμνθ- (helminth) meaning "worm." Platyhelminths are pretty amazing creatures that lack some of the features we multi-cellular animals find pretty important, such as a digestive system. And a respiratory system.
Now, there are many other creatures out there lacking those systems that make their livings as parasites, and while it's true that there are a lot of parasitic platyhelmithes, there are also a decent number of them out making a living for themselves just fine without these trivial systems that we might consider indispensable. They do it by having a very simple body plan. It has to be, because the only way for oxygen and nutrients to reach all their cells is by diffusion, which has practical limits on how far it can work.
Within the platyhelminths, there is an order called Tricladida, whose members are commonly known as planarians. They have some remarkable regeneration abilities; some of them reproduce by simply leaving their tails behind, which then grow into a whole new planarian! In fact, cutting them into pieces merely causes all the various pieces to regrow into new flatworms. There have even been experiments performed where people cut a planarian's head in half longitudinally, and both halves regrew into new complete heads, leaving a two-headed planarian! (I have never heard if anyone ever took this to the logical next step and created a nine-headed hydra planarian.)
Planarians, like the rest of the platyhelminths, are mostly very small creatures, a few inches long at the most. This is due to their simple body plan as mentioned above. But there is one species, Bipalium kewense, that grows much longer than that, reaching up to 60 cm (about 2 feet) in length! What's even more interesting is that this particular species lives on land rather than in water...and it lives in Hawai‘i. (They also live in many other parts of the world besides Hawai‘i, having been inadvertently introduced around the globe by humans. They are believed to be native to Southeast Asia, however.)
Often while walking in the early morning I would see these strange, worm-like creatures sliding along across the sidewalk without knowing what they were. Recently though, I learned that B. kewense was the largest known planarian in the world and realized that was what I had seen all those times, at which point I thought it would be neat to get some pictures or maybe a video of one of them.
With that goal in mind, I got up early one morning this past week and set out to look for them before going to work. After a lovely early-morning walk of twenty minutes' duration along the sidewalk next to the uncleared jungle on the UH campus, I found what I was looking for, a B. kewense a good eight inches long out for an early-morning squirm.
|Bipalium kewense on a sidewalk in Hawai‘i.|
(I discovered that it's really difficult to hold a phone in one hand to take pictures while trying to position your other hand and an oblivious piece of wildlife in a nicely framed photograph taken by that same phone.)
You may be wondering about the shovel-shaped appendage on one end of this particular flatworm. That's its head, and it contributes to their endearing common name of "hammerhead slug," despite them not being related to slugs at all (other than both of them being in the kingdom Animalia). Here's a closeup if you want to see it better:
|Close-up of the head of B. kewense, also called the "hammerhead slug."|
I don't think the reason for the head's peculiar shape is exactly known, but since they are known to eat earthworms, it may serve as a method to increase the area of their sensory apparatuses, allowing them to better find and follow earthworm tracks.
Finally, if you'd like to see one of these strange creatures in glorious motion, you're in luck! I took a video (if you don't care enough about the planarian to watch the video, there's also some nice early-morning birdsong recorded in it):
All in all, planarians are fascinating creatures, and I think it's really neat to be able find them in the wild around here.