These are just very small flat worms, and more common in fresh water than you may think. In the wild, flatworms that live in water avoid the light, so they are usually found attached to the under-side of stones or twigs, and they will often be in groups. They typically have a pointed tail-end and an arrow-shaped 'head' end, with two very primitive eyes, little more than two light-sensitive patches. Most of them are less than 10 mm long.
A land planarian looks sometimes looks like this, with a sort of "shovel" head. For some odd reason, people don't call them "land flatworms". Posh people call them terrestrial planarians. (I put that in there for the search engines).
Land planarians have a much longer body, up to 300 mm long. They also have a shovel-shaped head, and they travel on a layer of slime like a snail or a slug. The slime is amazing stuff, because it hardens, and if a land planarian goes over a drop, the one species I have played with is able to lower itself down on a thread of hardened slime, like a spider descending on its web. The body gets much longer when it is stretched by gravity as it dangles. On the flat, they are about a third as long.
Land planarians will eat earthworms so they can sometimes be a problem in worm farms (which means you may find them in a worm farm!), but they also eat a wide range of other garden 'meat', including slugs. If you leave a small piece of meat in some leaf litter, inside some sort of cage to stop larger animals getting at it, you may attract a land planarian, though this has never worked for me. This one was on a brick wall and quietly consuming a small millipede. For scale, the picture covers a space about 10 mm high.
Notice the slime here (if you click on the picture, you should see it full-size). I think this is how they digest their food.
The last reliable count that I have seen, made in 1999, showed 822 species of terrestrial flatworm around the world. The world's greatest variety is seen in mainland Australia, Tasmania and New Zealand.
But why are flatworms flat? Probably, so they can breathe! With no lungs, blood or circulation, oxygen has to diffuse in through their skin to reach all of their cells, but the gas is all used up before it gets more than about 0.5 mm from the surface, limiting their thickness.
Flatworms will regenerate (grow back missing bits) if you chop them in halves through the middle with a sharp razor blade. It is also possible to split the "head" lengthways between their primitive eyes, and produce a two-headed flatworm. This requires a sure hand, and it is unkind. Both ends of a cut worm will grow the missing end, and both ends of a trained worm were once supposed to recall a lesson taught to the whole worm, though most people no longer believe this.
Flatworms are photonegative (light-avoiding) so they need to be kept in opaque containers with covers. With the water ones, change the water regularly, feed them once a week on chopped raw beef liver or hard-boiled egg yolk, and take out the left-over food with a pipette after a few hours. Apparently they do not often reproduce sexually in captivity, but do occasionally produce orange "cocoons" that hatch out a month or so later. They also reproduce by fragmentation, even when nobody chops them up.
I have caught aquatic planarians in the past by scooping up some drain mud and putting it in a white dish, then waiting for the mud to settle, at which point, they leave tracks in the mud that you can see. Happy hunting!
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This blog covers quite a few different things, so I tag each post. I also blog about history, and I am currently writing a series of books called Not your usual... and the first two have been accepted by Five Mile Press, The offcuts appear here with the tag Not Your Usual... . For a taste of Australian tall tales, try the tags Speewah or Crooked Mick. For a miscellany of oddities, try the tag temporary obsessions. And language us covered under the tags Descants and Curiosities, while stuff about small life is under Wee beasties.
