Animals like annelids, arthropods, etC), where the body can be divided into identical left and right halves in only one plane, exhibit bilateral symmetry.Animals in which the cells are arranged in two embryonic layers, an external ectoderm and an internal endoderm, are called diploblastic animals, e.g., coelenterates. An undifferentiated layer, mesoglea, is present in between the ectoderm and the endoderm