The Platyhelminthes (Flatworms)

Major Characteristics Animal Characteristics Flattened dorsoventrally Bilateral Symmetry Triploblastic Acoelomate Incomplete Gut

Body Plan: External Bilateral symmetry divided these organisms into distinct dorsal, ventral, anterior, and posterior portions Such symmetry has lead to cephalization in which the planaria have specialized senses in the anterior portion of their bodies

Body Plan: Internal Platyhelminthes are composed of an ectoderm, mesoderm, endoderm making them triploblastic They contain no cavity or coelome and are thus acoelomate

Movement Planaria move using cilia and muscular undulations through a mucus that aids in the adhesion and traction of the cilia Many flatworms are parasitic and don’t move

Feeding Unlike most bilateral animals the mouth of the planaria is located ventrally and is incomplete (without an anus) They feed through a retractable pharynx and digestion occurs in a distributed digestive cavity

Reproduction Asexual reproduction occurs by transverse fission Sexual flatworms are monoecious producing both sperm and eggs Fertilization occurs internally and the offspring develops externally and proceeds through a larval stage