The Class Turbellaria contains mostly free-living forms ranging in size from a few mm to 50 cm. Most species are bottom dwellers in marine and freshwater environments that crawl over rocks, sand or vegetation. Smaller forms can swim by means of ventral cilia, but more often they move by laying down a sheet of mucus that aids in adhesion and helps the cilia gain traction. Larger forms use powerful muscle contractions to crawl or swim. Unique to turbellarians are rod-shaped rhabdites found among the ventral epidermal cells of the body surface. These rhabdites secrete mucus that coats the animal's body, possibly for protection against predators or to prevent drying.

In terms of nutrition, most turbellarians are predators and scavengers. Epidermal mucous secretions trap and kill prey items. A muscular pharynx everted though the ventral mouth is used to secrete digestive enzymes into prey, which is then sucked into the branched intestine that forms a gastrovascular cavity. In addition to a simple nervous system, turbellarians have light-sensitive eye spots called ocelli that help orient the animal to the direction of light. Touch and chemical receptors in some forms like the planarian seen in lab are concentrated in lateral projections from the head called auricles that look like ear lobes. Reproduction in turbellarians can occur asexually through fission or sexually; all forms are monoecious but practice cross-fertilization. Planarians are also known for their tremendous powers of regeneration, and a planarian that has been cut into three pieces will give rise to three new complete individuals!