In terms of gas exchange, internal transport and excretion, flatworms have a high enough surface area to volume ratio to accomplish these tasks by simple diffusion.  Nitrogenous wastes (mainly ammonia) diffuse out but protonephridia (absent in marine forms) may rid the body of some other wastes as well as aid in osmotic regulation by pumping out excess water. Note: It was the development of these protonephridia that allowed turbellarians to invade freshwater habitats!  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 (most turbellarians are negatively phototropic and move away from light). Touch and chemical receptors in some forms like the planarian seen in image above 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!