Turbellarians (Planarians; Free-Living Flatworms)
Turbellarians have primitive body plans. There are many species, but all have some traits in common. In Missouri, most are tan, brown, black, or gray (but they can be brightly colored elsewhere). They are flat or ribbonlike. Their bodies are bilaterally symmetrical: They have a left and right side, and what amounts to a back, a belly, and a head (they are not radially symmetrical like starfish and sea anemones).
Their simple nervous system is concentrated at the head end. Often there are 2 earlike flaps on either side of the head. There are often 2 primitive eyespots on the top surface of the head; they sometimes appear “cross-eyed.” The mouth is located about midway down the underside of the body; this is the only opening to the digestive tract. During feeding, a muscular, tubelike pharynx extends out from the mouth and sucks food into the creature. Tiny hairlike cilia on the underside, a thin layer of secreted mucus, and subtle muscular contractions account for their gliding movement.