Peter J. Hayward

Flatworms and Ribbon Worms

This cross section of the simple flatworm body illustrates a major difference between the Radiata and the Bilateria. Whereas the Cnidarians and comb jellies had an inert layer of mesoglea between the epidermis and gastrodermis, The Bilateria have a region of packed cells that constitutes a third cell layer. From these cells, muscle tissue developed as well as a variety of specialized organs. Such structures could not form from epidermis, which is designed as an interface between the animal and its environment or from the gastrodermis that is specialized to keep digestive juices with the digestive cavity and to promote digestion and food distribution. A major step in establishing the Bilateria body plan occurred when the early embryo of these animals acquired a middle layer of mesoderm. The outer embryonic layer, ectoderm, developed into the epidermis and the inner endoderm layer into the gastrodermis as occurs in all Eumetazoans, but the mesoderm gave rise to the all important third layer of cells that is not present in the Radiata.