Flatworms Class Turbellaria, Class Monogenea, Class Trematoda, Class Cestoidea

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.