A major difference between platyhelminthees and coelenterates is that platyhelminthes

A major difference between platyhelminthes and coelenterates is that platyhelminthes have developed a mesoderm. The mesoderm is a middle layer of cells that lies between the endoderm (innermost layer) and the ectoderm (outermost layer) in most triploblastic animals. Platyhelminthes are triploblastic animals, which means they have three germ layers (ectoderm, mesoderm, and endoderm). In contrast, coelenterates are diploblastic animals, which means they have only two germ layers (ectoderm and endoderm) and do not have a mesoderm. The development of a mesoderm is an important evolutionary advancement because it allows for the development of more complex organs and systems in animals, such as muscles, circulatory systems, and kidneys. Therefore, the development of a mesoderm is a significant difference between platyhelminthes and coelenterates.