Flatworms are simple white or brown flattened worms and move about their damp habitat by a series of undulations. Most flatworms are free-living in streams and pools, but many are parasitic. Although simple they have a bilateral nervous system and the head end of some species even has a rudimentary brain to integrate signals from eyespots. Some have the ability to continue to grow as two flatworms when cut in half - the bit that had the tail grows a new tail and its tail becomes the head.

Those in the caves are small (about 2cm long or less) and some examples found in Mendip are:
Phagocata vitta a small white species, recorded from Fairy Cave and Rickford Farm Cave
Dendrocoeleum lacteum a larger, white, predatory species, recorded from Stoke Lane Slocker
Crenobia alpina, a dark-coloured species, recorded from Eastwater, Lamb Leer Cavern and Swildon’s Hole
Polycelis felina, another small, dark species recorded from St. Cuthbert’s and the main streamway in Swildon’s Hole.
An unidentified Typhloplanid species of the order Rhabdocoela has been recorded from Aveline’s Hole.