Response to Environment

Free-living flatworms have a definite head in which a simple brain is located. This brain is the control center of a simple nervous system that stretches throughout the body. Many flatworms have one or more pairs of light-sensitive organs called ocelli, or eyespots, that detect light. Most flatworms have cells that are sensitive to chemicals found in food, and other cells that tell the worm which way the water is flowing. The nervous system helps a flatworm find food and dark hiding places.

Parasitic flatworms often do not have much of a nervous system because they are protected by their host, though the host may not want to.
The nervous system in tapeworms has completely disappeared as the worms have adapted to their parasitic lifecycle.

Roundworms have simple nervous systems. They have several ganglia in the head region, but lack a real brain. They possess several types of sense organs are simple structures that detect chemicals given off by prey or hosts. Several nerves extend from the ganglia in the head and run the length of the body. These nerves transmit sensory information and control movement.