Planarian Worms

Once the wet weather starts, the Museum often gets inquiries about small, brightly coloured "snakes", invasions of "leeches", unusual "slugs" and similar queries - all relating to an animal which is actually none of these.

The culprit is any of several species of terrestrial planarian worm. The most common of these is called the Blue Planarian, Caenoplana coerulea. It’s about 6 cm long and a deep navy blue with a pale stripe down its back, bright blue underneath and a pink- or red-tipped head end.

After wet weather these exotic-looking flatworms can emerge in large numbers. Part of the confusion in the public mind is based on their appearance and partly on their movement. Being flatworms they move on a slime track like a snail or slug’s. I suppose if you have a good imagination they could resemble small snakes as they don’t contract and elongate as an earthworm does. Their long slimy shape and prominent stripe superficially resembles land leeches but their action on land is very unlike the leech’s ‘inchworm’ or looper caterpillar action.

So what are they and what are they doing? These terrestrial planarians are related to the flatworms much talked about in biology lessons as ‘super regenerators’ – chopped into tiny pieces each has a good chance of regenerating into a new worm;slit the head lengthways and the worm will grow two heads, and so on. These land-dwelling relatives are larger than those in the biology lab but just as able – quite a few species seem to reproduce by fragmentation and subsequent regeneration.

Interestingly they are all predators, and the wet weather brings them out hunting. Blue Planarians use their viscous slime trails to trap small animals like slaters and millipedes. The planarian cruises along old trails and drowns any victims in slime before sucking out their insides with a mouth located on its belly. If this all sounds a bit ugly, consider that the planarian digestive system has only one orifice – all wastes must exit the same way the food went in, via the mouth.

So the next time you see this flatworm or one of its relatives you’ll know what it is, and you may wonder if it knows whether two heads really are better than one.