Flatworms

Flatworms are small, multicelled animals with elongated bodies that have clearly defined anterior (front) and posterior (rear) ends. These worms are bilaterally symmetrical, meaning that their two sides reflect each other. They usually have a recognizable head, which houses gravity and light-receptive organs, and eye spots. They lack circulatory and respiratory systems and have only one opening that serves both as their anus and mouth. Most flatworm species live in fresh and marine waters, although some live on land.

Their soft, flattened bodies are composed of three layers-the ectoderm, endoderm, and mesoderm. They may be covered by a protective cuticle or by microscopic hairs, called cilia. Their internal organs are comprised of a nervous system, usually hermaphrodite sexual organs, and an excretory system.

As members of the phylum Platyhelminthes, flatworms belong to four classes: Turbellaria, Monogenea, Trematoda, and Cestoidea. Within these four classes, there are hundreds of families and some 10,000 species, including animals with common names like free-living flatworms, parasitic flatworms, tapeworms, and flukes.

Planarians

Probably the most familiar Turbellarians are the planarians, soft-bodied, aquatic, flattened worms that appear to have crossed eyes and ear lobes. In fact, the crossed eyes are eye spots with which the worm can detect light. The lobes to each side are sensory and also are equipped with glands to secrete an adhesive substance used in capturing prey.

The single opening on the ventral (bottom) surface of the worm serves as both mouth and anus. Internally the worm has a complex, branching gut that courses nearly the full length of the body. Since the worm has no circulatory system, the elongated gut brings food to nearly all areas of the worm's body. Planaria have no skeletal or respiratory systems.

These animals possess great powers of regeneration. If a planaria is cut in half, the front half will grow a new tail section and the rear half will generate a new head. If cut into thirds, the middle third will regrow a head and tail and the other two sections will regenerate as described.