Flatworms are simple white or brown flattened worms and move about their damp habitat by a series of undulations. Most flatworms are free-living, 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 are:

Polycelis felina (found in Agen Allwedd), Phagocata vitta and Crenobia alpine (both found in Porth yr Ogof) all prefer mud and sluggish water. Dendrocoelum lacteum (found in Dan yr Ogof) is relatively big and fast (for a flatworm) and it can capture Proasellus cavatiticus, they are often found together. Some leave a slimy mucus trail to trap their prey.

Flatworms can be seen in Porth yr Ogof in the muddy gour pools at the bottom of The Creek.