| Germ Layer | Tissue types |
| Ectoderm | Outer layer of skin, nerves, brain, cornea and lens of the eye |
| Mesoderm | Connective tissue of skin (dermis), bone, muscle (including cardiac muscle), cartilage, blood cells and blood vessels, fat cells, reproductive tract |
| Endoderm | Internal lining of organs of the digestive tract, internal lining of respiratory tract, liver |
Presence or absence of a coelom
Triploblasts may develop an internal body cavity lined with cells derived from mesoderm, called a coelom (pr. see-LŌM). This epithelial-lined cavity is a space, usually filled with fluid, which lies between the digestive system and the body wall. It houses organs such as the kidneys and spleen, and contains the circulatory system. An Organ is a differentiated structure that performs a specific function in an organism; it consists of many cells and of various tissue types. Triploblasts that do not develop a coelom are called acoelomates , and this internal region is completely filled with tissue, although they have a gut cavity. Examples of acoelomates include the flatworms. Animals with a true coelom are called eucoelomates (or coelomates) ( [link] ). A true coelom arises entirely within the mesoderm germ layer. Animals such as earthworms, snails, insects, starfish, and vertebrates are all eucoelomates. A third group of triploblasts has a body cavity that is derived partly from mesoderm and partly from endoderm tissue. These animals are called pseudocoelomates . Roundworms are examples of pseudocoelomates. New data on the relationships of pseudocoelomates suggest that these phyla are not closely related and so the evolution of the pseudocoelom must have occurred more than once ( [link] ). True coelomates can be further characterized based on features of their early embryological development.
Protostomes and deuterostomes
Bilaterally symmetrical, triploblastic eucoelomates can be divided into two groups based on differences in their early embryonic development. You may have noticed in [link] the terms "Protostomia" and "Deuterostomia". Protostomes (members of the Protostomia) include phyla such as arthropods, mollusks, and annelids. Deuterostomes (members of the Deuterostomia) include the chordates and echinoderms. These two groups are named from which opening of the digestive cavity develops first: mouth or anus. The word protostome comes from Greek words meaning “mouth first,” and deuterostome originates from words meaning “mouth second” (in this case, the anus develops first). This difference reflects the fate of a structure called the blastopore ( [link] ), which becomes the mouth in protostomes and the anus in deuterostomes. There are other developmental differences between protostomes and deuterostomes, including the mode of formation of the coelom and the pattern of early cell division of the embryo.