Phylum of flat soft-bodied invertebrates that include tapeworms and flukes. The flatworms are free-living as well as parasitic and lack respiratory, skeletal, and circulatory systems. The body is not divided into true segments, but rather has spongy connective tissue called parenchyma filling in the space between organs. Flatworms are hermaphroditic and are the lowest invertebrates to possess three embryonic layers of endoderm, mesoderm, and ectoderm. Bilaterally symmetrical worms of the phylum Platyhelminthes, having a soft, solid, usually flattened body without an internal cavity.
