Platyhelminthes are the ribbon and leaf-shaped flatworms. The soft body of the flatworm is bilaterally symmetrical. Structures for capturing and consuming prey are localized in the anterior end except in turbellarian flatworms, in which the mouth is ventrally located. Flatworm organs are composed of tissues and are organized into systems. Flatworms have distinct mesoderm tissue in comparison with the gelatinous mesoglea middle layer of cnidarians and ctenophores. The platyhelminth, like the cnidarian, lacks an anus. The flatworm middle tissue layer - a loose mesoderm called parenchyma - never splits into a cavity (coelom) in which internal organs are suspended. Flatworms and other animals without a coelom are called acoelomates. Flatworms - having three tissue layers - are triploblastic, have spiral cleavage of their eggs, and are among the least complex of bilaterally symmetrical true metazoans.