After fertilization the zygote divides by cleavage process to form blastomere. Blastomere cells are arranged to form a distinct morula. Later the cells of the morula dasa give rise to the hollow blastula. Blastula cells develop into bilayer or trilayer gastrula. Scientist Heinz Christian Pander discovered the embryonic layer. Multicellular organisms are divided into two groups based on the embryonic stage.
(i) Diploblastic animal: Those animals which have two layers in the embryo are called diploblastic animals. Their embryos have two layers called ectoderm and endoderm. Between the ectoderm and endoderm is an acellular layer called mesogia. Animals in the order Cnidaria are bivalves. Such as Hydra (Hydra vulgaris), Jellyfish (Aurelia aurita), Obelia (Obelia geniculata) etc.
(ii) Triploblastic animals: Animals which have three layers in their embryo are called triploblastic animals. The outer layer is the ectoderm, the middle layer is the mesoderm and the inner layer is the endoderm. The order Platyhelminthes, Nematoda, Mollusca, Annelida, Arthropoda, Echinodermata and Chordata are phyla.