Unsegmented worms are the simplest animals with bilateral symmetry to have a digestive system. Most of the members exhibit enough cephalization, or development of the anterior end. Unsegmented worms can be separated into two major phyla: phylum Platyhelminthes and phylum Nematoda. Free-living and parastic flat worms belong to phylum Platyhelminthes and round worm belong to phylum Nematoda. (Also phylum Rotifera)
Characteristics
Phylum Platyhelminthes is also known as flatworms because bodies are flatten dorsal-ventrally. Flatworm is acoelomate and contains three body layers: outer Ectoderm, middle Mesoderm, and inner Endoderm
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