Germ layers are primary layers of cells, which differentiate in the embryo at the time of gastrulation. All the tissues and organs develop from the germ layers.
The animals having two germinal layers, viz, ectoderm and endoderm are called diploblastic, i.e., porifers and coelenterates. Animals with three germinal layers, viz., ectoderm, mesoderm and endoderm are called triploblastic animals, e.g. platyhelminthes to chordates.
In animals with diploblastic organization there is a single cavity surrounded by body wall. This cavity is termed as coelenterons or called as gastro-vascular cavity. It opens outside by a single aperture situated at the anterior end of the animal. Although there is no true, alimentary canal and no true body cavity (coelom), but this cavity serves well as digestive cavity as well as vascular system. The digested products of good are circulated through out the body wall.