In the Animal Kingdom, most phyla have three primary tissue layers: a ectoderm, mesoderm, and endoderm. The ectoderm is the outside layer, the mesoderm forms the wall of the body cavity (coelom) and the endoderm forms the internal organs. This is called triploblastic. The three layers are especially clear during embryological development.

Other phyla have just two primary tissue layers: an ectoderm and an endoderm. These are the phyla Cnidaria and Ctenophora. Cnidarians include jellyfish, corals, sea pens, sea anemones, sea pansies, sea wasps, and sea fans, while ctenophores are referred to as comb jellies. These simple animals lack body cavities and true organs. They are referred to as diploblastic because of their dual-layered nature.

Triploblastic animals are believed to have evolved from diploblastic animals between 580 and 650 million years ago. The body cavity may have evolved from the stomach pouch present in jellyfish. Very early in the fossil record (between 600 and 542 million years ago), diploblastic animals far outnumbered triploblastic animals, but triploblastic animals overtook the diploblasts during the time of the Cambrian explosion (~540 million years ago) and have remained dominant since.

In addition to triploblastic and diploblastic animals, there is one group of animals, sponges, which have only a single, largely undifferentiated tissue layer, making them monoblastic. For many decades, it was thought that sponges represented the most basal of animal groups, but detailed genetic studies in 2008 revealed that sponges were in fact secondarily simplified, and cnidarians were in fact the most basal of known animals.

Triploblastic animals have become dominant on this planet because of the advantages afforded by having a body cavity. Having a body cavity protects the organs from external pressure, cushioning them in fluid. it also provides a hydrostatic skeleton for muscles to pull against, increasing the possibilities for strength and speed. The body cavity also allows internal organs to develop at separate growth rates than the outer tissue layer of the body. Every complex animal is a triploblast, but some triploblastic animals have become secondarily simplified and lost their body cavities. These animals are referred to as acoelomate.