Animals embryonic body layers (Germ Layers) Or Classification on basis of Tissue Organization

Animal body plans also vary with regard to tissue organization. All animals, the embryo becomes layered during gastrulation. As development progresses, these layers, called germ layers, form the various tissues and organs of the body. Ectoderm, the germ layer covering the surface of the embryo, gives rise to the outer covering of the animal and in some phyla, to the central nervous system. Endoderm, the innermost germ layer, lines the pouch that forms during gastrulation (the archenteron) and gives rise to the lining of the digestive tract (or cavity) and to the lining of organs such as the liver and lungs of vertebrates. Cnidarians and a few other animal groups that have only these two germ layers are said to be diploblastic. All bilaterally symmetrical animals have a third germ layer, called the mesoderm, which fills much of the space between the ectoderm and endoderm. Thus, animals with bilateral symmetry are also said to be triploblastic (having three germ layers). In triploblasts, the mesoderm forms the muscles and most other organs between the digestive tract and the outer covering of the animal. #bodylayers