Mesoglea is the undifferentiated layer which is present between the ectoderm and endoderm. Coelenterata and Ctenophora show the scattered pouches in between ectoderm and endoderm. These groups have true body cavity. They are Pseudocoelomates. However, this layer is not present in Platyhelminthes. In these animals, the body cavity is absent. They are also called as acoelomates.Thus, the correct answer is 'Platyhelminthes.'