At a tissue level, an adult hydra is essentially a tube composed of two tissue layers, the ectoderm and the endoderm, which extend throughout the animal.
The branchial arches begin as cylindrical cores of mesenchyme sandwiched between continuous sheets of epidermal ectoderm and internal endoderm.
It is involved in mesoderm development and differentiation of the neural ectoderm into epidermoblasts and neuroblasts.
The anthozoans are diploblastic animals developing from two epithelial germ cell layers: the ectoderm , facing the seawater, and the endoderm, facing the gastrovascular cavity or coelenteron.
The presumptive neural tube tissues consist of a layer of ectoderm along the dorsal midline of the embryo, between the notochord and an outer layer of epidermis.
As the neural tube closes, the ectodermal cells continue to grow and divide very rapidly.
In most polychaetes, the ectodermal and mesodermal derivatives of the growth zone become incorporated into segmental tissues in an anterior-posterior sequence.
At the very tip of the limb bud is a thickening in the ectoderm - the apical ectodermal ridge or apical ridge, which runs along the dorso-ventral boundary.
For example, chaetal sacks are one of the few ectodermal features of polychaetes that regularly express engrailed, and chaetae are the primary indurated elements in the ectoderm.
The term iteration is used to denote processes that result in both metameric segments and repeated ectodermal skeletal structures.