When any plane passing through the central axis of the body divides the organism into two identical halves. Example- Coelenterates, Ctenophores etc.
When the body can be divided into identical left and halves in only one plane. Example- Annelids, Arthropods, etc.
When any plane that passes through the center does not divide them into equal halves. Example-Sponges.
Animals in which the developing embryo has a third germinal layer, mesoderm, in between the ectoderm and endoderm. Examples- platyhelminthes to chordates
Animals in which the cells are arranged in two embryonic layers, an external ectoderm and an internal endoderm. Example-coelenterates