Classification of Animal Kingdom on basis of Arrangements of cells in germ layers
Some organisms have two kinds of tissues that develop in embryonic life. These tissues are the primary germ layers.
Diploblastic: The organism has two germ layers; an external ectoderm and internal mesoderm. An undifferentiated layer of cells called mesoglea is seen in between these two layers. Eg: Coelenterates.
Triploblastic: The organism has three germ layers, the third germ layer being the middle mesoderm. E.g. Platyhelminthes to chordata.