Members of the phylum Platyhelminthes are flatworms, such as the planarian. Grubs and tapeworms are other examples of flatworms. Flatworms display bilateral symmetry; that is, the left and right halves of the body are mirror images of one another. Another characteristic of the platyhelminthes is cephalization, which means that one end of the animal functions as a head. The “head” contains a mass of nerve cells that acts as a brain and specialized regions for sensing light, chemicals, and pressure.

Flatworms have three distinct layers of tissue, all composed of living cells. They have true organs and organ systems for digestion, movement, excretion, and reproduction. The digestive system consists of a muscular tube with one opening at the mouth. The excretory system consists of a network of water-collecting tubules that empty their contents into sacs leading to the exterior.

Movement occurs by the contraction of muscle cells that lie below the epidermis. The contractions are coordinated by signals from a nervous system. Most platyhelminthes have both testes and ovaries, and the organisms pair up to exchange sperm and eggs in fertilization.