Stare into a pond and let your gaze fall on the silty bottom and you might notice tiny, slate-grey slithers of animal life. They’re not obvious at first as, being just a few millimetres long, they are easily overlooked. Even a water flea oozes more charisma than a flatworm.
But don’t ignore flatworms, of which Britain has about 12 freshwater species, on account of their initial appearance. There are few pond creatures quite as bizarre as the class Turbellaria.
What are Turbellaria flatworms and what do they look like?
You really will need a hand lens to appreciate the turbellarians’ world of weird. The first thing to notice as you peer at their flattened form is that flatworms have a definite front end, and from the middle of their arrow-shaped head two eyes disconcertingly stare back at you. These simple eyes, or ocelli, are black but sit in a thinner window of skin, making their owners look a little boss-eyed.
If eyes determine the front of an animal, you might expect to find an anus at the posterior – but no matter how hard you look, you won’t find anything: flatworms don’t have one. Search for a mouth and you’ll not find that in the expected place either, for in most flatworm species it is located in the centre of the body.
How do flatworms reproduce?
The most chimerical quality of flatworms, however, is how they reproduce. Among their number most are hermaphrodite: not unusual at their end of the evolutionary tree. It’s their ability to repair and regenerate that truly amazes. A myth about worms, and earthworms in particular is that, if divided, each end will create a new animal. Sadly, it isn’t the case. But it is with flatworms.
Dice a flatworm any which way and each fragment will in time become a fully functional creature. How flatworms do this is not fully understood. But it’s a quality that has given them fame for being, in the words of 19th-century naturalist John Graham Dalyell, “immortal under the edge of a knife”.
What do Turbellaria eat?
The mouth is a proboscis-like arrangement, like having a hosepipe as a navel. Flatworms slowly groove away on the bottom of the pond, sucking food up through this mouth. Any waste products simply dissipate through their foliose bodies.
There is little as sensuous as the movement of a flatworm; they seem to float, levitating across the bottom of the pond. It’s an effect made possible by thousands of microscopic hairs called cilia that waft the animals along on a trail of mucous, and in doing so stir the water. Similar cilia are also used to set up feeding currents to pull food (fine detritus, algae and protozoa) into their mouths.
What does Turbellaria mean?
‘Turbellaria’ is derived from the Latin turbella, meaning to create a small commotion or turmoil, a name that doesn’t initially seem to fit such unassuming beasts. Yet it isn’t a reflection of their character, but of their ability to create a micro-storm of water currents. Many of the flatworms’ other names likewise refer to their bizarre qualities. ‘Planaria’, often used to describe all free-living flatworms, means to ‘lie flat’, while ‘triclad’ refers to those species with three dead-end gut passages radiating from the central mouth.
