Chances are that when I bring up the term Planaria at least some of you recall memories of biology class. If you didn’t actually do it yourself, the rest of you saw illustrations showing experiments in which these fleshy little flatworms were sliced and diced for the sake of science. Cut one in half and each half eventually grew a new half to create two whole individuals. Cut off its head and the severed head grew a new body while the headless body sprouted a new head – again forming two new entities. Cut only the head in half and the beast will end up with two complete heads on one body. There is no end to the regenerative abilities of these tiny fellows (although I don’t think a blender-treated worm has much of a chance). One nineteenth century scientist stated that they were “immortal under the knife.”
They still figure heavily in major research today, but you are free to admit that it was the freakish nature of this beast that fascinated (and still fascinates) us. If you’ve ever had someone look at you like you had two heads, then you can relate to the meek little planarian.
What is less known is that Planarians are actually very common members of our local aquatic community (and by that I don’t mean the YMCA). There are thousands of different kinds and few of them are actually called Planaria. That is a genus name of one particular group that has become a general term to cover the whole group of flatworms called Turbellarians. They go by names such as Dugesia, Girardia, and Schmidtea but we commoners just call them Planaria and we “is o.k. wid dat.” Should I say this amongst a group of flatworm scientists I would undoubtedly be looked at as if I had two heads.
So, why you ask do I bring this up? Well, I encountered a whole school of tubellarians the other day in a small drainage outlet. A thin sheet of ice covered the slowly flowing water and the flatworms were crawling about the bottom as if it were a balmy summer day. I bent down to get a better look and spotted at least a dozen at or near the opening of the drain pipe. It is not unusual to spot one or two, but to see a whole bunch of them in the middle of winter is worthy of note. I here, therefore, note.
I wanted to get an even better look and decided to de-glove and plunge my hand into the icy water to get one. They retracted into a ball up upon being touched. Using a piece of the fractured ice, I was able to convince one to drift up and re-attach to its surface. It adhered to the ice after I brought it out of the water.
A planarian on ice is a remarkable sight for two reasons. First of all, you can notice the basic body plan of these ditch worms. Many planarians have an arrow-shaped head with two side projections called auricles (they are not ears but look as if they could be). This type had a rounded head with a cleft palate. Look closely and you’ll see the trademark cross-eyed look of all planarians (see detail here). These are photo-receptive cells called ocelli and for all intents and purposes they are eyes (again, I am getting that look for those scientists!). The under view – as seen through the ice sheet (see below) – reveals a lighter spot located past the half-way point on the body which is actually the tubular mouth. These creatures feed on rotten meat, diatoms, and other invertebrates using this mouth. The other remarkable feature in this case was that this fleshy watery creature didn’t freeze solid out in the crisp winter air.
The planarian moved about on the ice as if it were a mere leaf or rock. It skated about on a layer of mucus and I assume this mucus, combined with a hefty internal dose of anti-freeze, is what kept this example mobile and care-free. I, on the other hand, was getting very cold. My exposed and ice-dipped fingers were beginning to lose their feeling, so I had to plunk the cross-eyed creature back into the drink and give my hands the blower treatment.
How incredible it is that the fine little things of nature are so very tough in so many ways.