Planarians are in the flatworm phylum, Platyhelminthes. Like all flatworms, planaria are bilaterally symmetrical which means that the left and right halves are mirror images of one another. The planaria nervous system is an anterior “brain” consisting of large ganglia. From the ganglia, two ventral nerve cords run the length of the body. Short transverse nerves connect the two ventral nerve cords like the rungs of a ladder.

planaria nervous system
Flatworms also have two eyespots called "ocelli". The ocelli are sensitive to light and are connected to the cerebral ganglia. Generally, the flatworm avoids light. The eyespots sense light but don’t see images.

Flatworms have "auricles" that project from the side of the head. These auricles contain chemoreceptors that are used to find food.Humans have chemoreception too—we sense chemicals using both smell and taste. The human nervous system is a more complex system that evolved from a simpler one like that found in planaria. This is part of the reason why studying planaria might help us learn about the human nervous system.

Planaria can be raised in the lab in an open beaker. They will eat raw liver that is tied to a thread and lowered into their beaker. The planaria will quickly find the food and then wander off again when they are full. Remove excess uneaten meat so that it doesn't spoil in the water.