1. The Platyhelminthes (Flatworms)

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

3. 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

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

5. 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

6. 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

7. 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